Check the harmonic balancer. The balancer on a 350 is smooth all the way around, while on a 400 it is machined part of the way around. 400sbc's and 454bbc's are externally balanced and to my knowledge all other chevy engines are internally balanced, hence the difference in the balancers. From what I've been told all 400 sbc's came with automatic trans. In order to run a 4 or 5 speed you will need a flywheel from a 454. I ran a 400 sb in a 62 pickup with a 4 speed and that's what I ended up having to do.