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Bottom Line Up Front: Can I use grade 5 washers and grade 8 nuts for the head and manifolds?

I have removed the head and manifolds. They have been sandblasted and will be resurfaced, primed (2000 degree) and painted (1600 degree). Since the nuts and washers will not be painted I'm thinking of using black oxide (900 degree). In black oxide, I have found grade 8 nuts but only grade 5 washers.

I don't know.

But, Max Merritt has head nuts and washers. They also have studs in case you have any that are damaged.

The engine probably has rolled threads where the studs go into the block so important to not run a tap into the holes or use modern over the counter studs. As for nuts and washers, Grade 8 wouldn't hurt but might be overkill and finding them with the 3/4" hex might be difficult as it's not a standard size for that thread as far as I recall. I think 11/16" is the standard size.

Regarding Max Merritt, I'm expecting their hardware will need to be painted but I'll ask. I'm not going to paint the nuts/washers after I install the head. I don't think I want SST either.

The head bolts and nuts were painted at the factory and to me they look better that way. Your choice, though.

I've found that the flat-to-flat is 11/16" but I don't see that as a problem. The height is 3/8" which matches what was on it. I thought about using a taller nut to offset the diameter but that's limited by the stud length.

The contact area is too small for cast iron even with washers so you really do need the 3/4" hex which is the reason it was used originally. 11/16" is all that I can find too so if you buy from Merritt, be sure to ask which theirs is! Tuscon Packards might have them too (possibly for a lower price) and I know they have the studs. Again, the studs have rolled threads where they go into the block so important which end goes in and which goes out and also important not to run a tap into the threads in the block.