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The stock manifold bolts are 3/8" x 16. Any set for a SBC will work (even a BBC - you just get 4 extra bolts). I used the stainless 12-pt jaunts from ARP.
EDIT: They are also 1 3/8" long. Remember several of these bolts in OEM form have a stud on top for mounting accessories. Using a SBC bolt you don't get the studs, obviously.
I used them when I did H/C, I bought a ARP engine bolt kit from a member on here that was still new in the box, had all the stuff in there. Made it nice because every thing is new and labeled.