Hello Mike,
It been a while since that engine was put together, if I did some digging it's possible to find the ARP invoice for the studs and nuts. It would be easier to measure the studs and order the lengths required. If the stock exhaust manifold is used, the long center exhaust manifold stud might not be available from ARP. They are all M8 x 1.25. Here is a link to ARP, look at page 75.
http://www.arp-bolts.com/Catalog/Catalog.html
As for Inconel hardware, these are not easy to obtain unless you're near what once was Boeing's surplus or So Cal or other places where there is a good amount of aero surplus available. Every so often this stuff appears on evil-bay. I purchase this stuff when available and squirrel it away in shop stock.
There are a number of aircraft hardware suppliers on line such as Coast Fab:
http://www.coastfab.com/
Genuine Aircraft Hardware:
http://www.gen-aircraft-hardware.com/
And many others... The cost might be a problem.
Of the A286 bolts that are some what common surplus, it would be the NAS1003 to NAS1020 series. This part has been around for a long time and shows up often enough surplus. Use this drawing as a guide to what might fit your needs.
Note 4 on sheet 2, shows a call out for this bolt to operate to 1200 degrees F.
Here is what a NAS1005 bolt and matching A286 silver plated nut looks like. The other bolt is a Boeing spec part (B30UU5)made of Inconel 718. Note the smooth, ground finish on the bolt's grip length.
Put these on the exhaust system and they won't rust, seize on, break off unless improperly sized or installed and makes the exhaust system easily serviceable for a very, very long time.
Keep in mind Bob's suggestion of using brass nuts is a good and very reasonable one and in many ways easier to do than source and modify the metric threads to inch standard which is what most aero spec hardware is.
Do you recall the lengths of the ARP studs you used?
Got a good source for Inconel fasteners in small quantities?
I've always thought of exhaust work as like plumbing repairs-- just about everything you touch needs to be replaced. Unless it was good stuff to begin with, that is.
///Mike.