I'm not sure if the aluminum block main studs are available separately from the block?

The #2, #3 and #4 main caps utilize a vertical 4 bolt mounting structure for added strength. All of the 5 main caps utilize the production cross bolts.
J 80-01-0001 M8 X 1.25 X 35 HEX FLANGE HD qty. of 10

Also the main studs and caps are a unique size for the aluminum 6.1 block casting. These will NOT fit any production cast iron cylinder block. I am pretty sure the 16 main studs and part numbers are:

H 80-03-0002 STUD - 7/16 BOTH ENDS - SHORT qty. of 6
G 80-03-0001 STUD - 7/16 BOTH ENDS - LONG qty. of 10

on the instructions .pdf and will have to email Prefix to confirm.