Was not a 3 speed manual trans standard on the 68 or 69 or 70, Roadrunner, 383 as standard engine. Floor shift as standard no doubt there. The same years Belvedere with a standard engine maybe a 318?? I bet the 3 on tree 3 speed trans as standard issue?????
I have an old Mopar bud that bought a 70 RR 383 3 on the floor car new back then right after HS. His dad would not let him order it with a 4 speed. He replaced the 3 with a 4 speed a week after getting it but he aid his dad never knew, If dad was around he always took off in second!! lol
PS Wayne or his son at Brewwers Performance can answer any question when it comes to what will work on any odd combo!