Get a '79-80 X1/9. From where Im sitting; its really the best bang for the buck (not to mention the best years for X1/9s, despite how others may try to say otherwise. You get an instant increase in TQ, which is really what these cars need. Stock for stock Im divided as to 1300 or 1500. Theyre both slow and weak as crap. One revs and sounds sweet and one can go on the highway without sounding like its going to explode, that's about the differences between the two, stock to stock. But no-one leaves either stock, and from that standpoint: the '79-80 is going to be the winner every time! Even with the later "Bertone" crowd pleading otherwise. The '79-80 years give you the best of all the X19 worlds. You get an updated yet still very italian interior. IMO The later cars interiors look more like a Hyundai or some other car trying to be something it isnt. You get the 5spd trans (even though I personally love the 4sd), you get the afore mentioned bump in TQ, added electronic ignition (no more points like your '78!!) and you keep the carbs!! The power potential is far better on a carbd 1500 then its later FI sibling (unless your working with the budget of Saudi Arabia or not staying naturally aspirated ), plus you get the ease of maintenance from carburetors. The Fi guys will attempt to scream "its more reliable.." ...
its not. If you have your carb set up properly, it will not need adjusting very often or at all, it will start and run everytime, and if something does go wrong, you can pop it off in under 5 minutes, have it completely stripped in another 10, and most likely figure out the issue, in another 10-20mins..... do a search on the issues the Fi guys run into Ahahaha! Get an early manual choke carb and you use it to tune your A/F mix and also aid in troubleshooting. Oh and when either system sets un used for long periods of time, they tend to not work well. Its just how things work. With a carburetor; there again, pop it off, replace a gasket and diaphragm, spray some cleaner and lube up some levers, then your off enjoying your car. Bosch jetronic does not like sitting and you can search through the archives of various issues and troubles. Not that the Fi is aweful, because its not. But the headache to performance to reliability equation, my experience is that the carb is far superior. Now speaking of fuel injection that doesn't like sitting un used; The old CIS system was so much worse, but thats different cars like VWs and Mercedes, and a different level of hell, which we wont go there. I love CIS btw, but Ill leave it at that. Really all this conjecture and opinion is arbitrary. You need to decide what you want from the car. As to your original question about having MWB build you a motor: sure, why not? Should you attempt to build one yourself? Sure, why not? You asked about dual carbs. Shamless plug, but Im currently selling a dual set up, for pretty cheap. Ive never run duals, so I cant speak as to reliability or what theyre like to live with. I would think the potential for less reliability and increased headaches is there, but I also would think that if set up properly, they should be just as fine as a single, except sound better and convert fuel to noise in a much more rapid fashion.
PS: Sell the '78 and get a '79-80