I would really like to buy high quality bolts. But ARP doesn’t seem to offer a complete kit and from checking the original replacement manifold 4 different bolt lengths are required beside the 2x2 washers. The only kit apparently right - according Firebirdcentral - is this one: https://www.firebirdcentral.com/1967_1969_Ram_Air_Long_Branch_Exhaust_Bolts_p/hwr-2005.htm
Maybe somebody can also tell me the actual thread length of the bolts. As it appears the thread length is the same for all bolts, which, thus, only differ by the shaft length. I checked the holes on the LH exhaust manifold to cylinder head again and it could be that the bolts are only missing, since I’m able to stick something deeply into both holes. If they are actually only missing I could screw new ones in and replace only the RH exhaust manifold, which is cracked, with the engine in place. Addressing the leaking engine could then be done in a follow-up task.