I just scored a M21 Muncie for my elky, I have an old bellhousing I think its from a truck #6263756 I know its not right by the numbers, but will it work and will a 11 inch clutch fit inside? In the mean time i'll look for a 621.

Here's a pretty good article on Chevy bellhousings and such.

http://www.novak-adapt.com/knowledge/chevy_standard_shift_info.htm

Pretty much spells it out.

Thanks Jon, looks like it will work!