Using American bolts on any GM inline transmission mount after about 1978 will strip the mount threads. GM converted to metric mount bolts about 1978 even though most of the rest of the transmission bolts did not change until 700s and 4L80Es came out.
350s & 400s - Mount, speedo, and solenoid bolts went metric about 1978. All other bolts were American to end of production.
200s, 200-4Rs, 700s, and all later transmissions had all metric bolts.