Hi Kent, thanks for the comments/info. Here's what I can tell you after a week of investigating things. I've had the new 1438 motor and a bunch of parts for a long time and it's been a bit of a head scratcher to figure what the plan was.
Interesting on the PBS pistons. I used to have a new PBS bellhousing adapter (1300/124) and it had a casting flaw in a run of them. Can't remember exactly what (IIRC, wrong position for bolt holes into transaxle) and it needed additional machining and maybe welding. The 1500 rods (as well as 1100 & 1300) and 1608 crank rod journals are the same diameter and width.
Using the 1608 crank, I calculated that the stock 1438 compression would go from 9 to 10:1. I'm okay with that for myself and not sure if I'd want to deck the block or mill the head unless it was needed to clean or prep the surfaces.
The 1438 crank stroke is 71.5mm and the 1608 is 80.0. Stock 1438 rod length by my measurement is 129.5mm (center to center), the 1500 rod is 128.5mm and the 1116 (and the 1300) is 120.0mm (way too short). The compression height (center of pin to top of piston) of the stock 1438 piston is 37.8mm and for the 1116 piston is 34.7mm. So that means the distance from the centerline of the crank to top of pistons for stock 1438 is 71.5/2 + 129.5 + 37.8 = 203.05mm. For the 1608 crank, 1500 rods and 1116 pistons it is 80.0/2 + 128.5 + 34.7 = 203.2mm. I don't think 0.15mm can be an issue. Note that the 1116cc pistons have very slight crown in them and the compression height figure would be to the top of the piston. Good info. on pistons in different Fiat models can be found here
http://www.pistonespersan.com.ar/marcas/PERSAN_Fiat.pdf
One nice thing about the 1500 rods is that they are quite a bit lighter compared to 1116 ones at 620 vs 665 grams. The 1608 crank is waay heavier than the 1438 at 13,130 gr (28lb 15oz) vs 9,635 gr (21lb 4 oz.). Have been thinking about lightening the flywheel a bit to compensate but have read a few too many warnings about doing this to stock steel ones to make me unsure. I don't think it would need much lightening because although the 1608 crank is heavier, the centrifugal/rotational mass of a crank is smaller compared to the much larger diameter of a flywheel. A 1438 crank & flywheel has 10 mm bolts same as the 1608. For racing, the flywheels are sometimes doweled to the crank which I think I may do. Later 124 spider engines with their higher HP have 12mm bolts.
The 1608 crank counterweights his the bottom of the cylinder bores on #1 & #4 cylinders by a very tiny amount. A little work with a grinder will fix that easily. This was standard practice on 850 903cc blocks when installing 74mm cranks. The crank also slightly kisses the block at the oil pump and a little grinding will easily fix that too.
The piston pin diameter in the 1438 & 1116 pistons is the same at 22mm. Main journal size of the 1438 & 1608 crank is the same. The rod journal size and rod big end size are the same and bearings are interchangeable. Note that there are two oil feed holes in the 1500 rods and for bearings you want the ones with 3 holes in them.
I have a recollection that the stock oil pump could do with an upgrade to increase flow. It's possible that one from a Euro 131 with OHV has better flow and I have one in my parts pile which is why I might have bought it. An oil cooler might be a good idea and is easy to do. I have an alloy Abarth sump to put on the motor but the oil pump hits it and will need to remove some metal on the sump (it's pretty thick).
I'm planning to do a little porting and polishing of the cylinder head, nothing too serious. I assume larger valves would be possible but not sure if there'd be a worthwhile gain in HP. I have an intake manifold for single Weber DCOE (came with the car) and another for dual DCOEs. The plan is to use the dual carbs. Not the original setup on a 1300/124 but the engine isn't the original Abarth one. Also have the correct carb, manifold & air cleaner if ever wanted for originality. I have a new Delta cam with performance spec (a regrind of course), another ebay find that came from Richard Radford. It's marked "D-9 623" and I've lost the specs on it. Maybe Delta could regrind another to that spec? Another nice piece I have is a Marelli electronic distributor that I got from Italy. The 1st one I got from the seller was for a twin-cam motor and had to get him to swap it with the correct one. Not easy because he didn't know a word of english and I had to struggle with google translate, plus returning something to Italy is not an easy process through Customs. I have a set of Alquati outer valve springs that I got from Midwest Bayless and have a 1" bind height and 210 lb spring rate. I have a set of new Abarth 124/131 OHV valve springs that I won't need now.
Something I've just found and am unclear on is the clutches. The engine in my 1300/124 came with a stock 124 flywheel and the clutch is 180mm dia., the transaxle input shaft appears to be standard 850 (overall length could be different when I pull it out) and the pilot bearing in the crank is 850 dia. I have a 1300/124 200mm clutch disc, pressure plate and input shaft all of which are PBS. I also have a new 1300/124 disc that is 180 mm. Maybe the very early 1300/124s had a 180mm clutch and 850 input shaft? Greg Schmidt refers to the PBS shafts as being "850 at one end and 124 at the other". I like the 200mm clutch setup and will use that which will be good for higher HP anyway. The PBS pressure plate I have has reversed straps (also mentioned by Greg Schmidt) and has the straps re-attached with bolts and nyloc nuts which I am not sure is a good thing - may see if I can find a clutch shop to rivet them on. The clutch on my 1300/124 could be stock 124, I just don't know. I came across a reduction drive starter for the 1300/124 bellhousings some years ago on ebay. Just re-discovered it in the closet... Means I won't have to worry about using or restoring the original Scaglia one.
One fun thing about the 124 1438cc OHV engines is that they are very similar in design to 850 engines so many modification procedures & methods are common to both. Just need to get some bearings for the crank & rods and then I can take the block into the machine shop. Also need to get some non-rotational crank seals.
Besides the 1438 motor that I'll be stroking, I have a 1585CC Euro 131 OHV motor that I got from someone in New Zealand. Providing the 1438 motor works out, and I see no reason why, I'll be selling it. I have an NOS head, two sets of pistons and a few other new parts for it and also a cam & distributor out of a Fiat 242 van that are needed to reverse rotation. This is an alternative to stroking a 124 1438 pushrod motor but will not look quite as original to a 1300/124 due to having a belt drive arrangement for the cam.
