Flywheel ? for the /6 guys

I took apart 1965 D100 and 1968 D200 slant 6 to get parts for my 1967 W200 (Small block V8). In both slant 6 engines, the bell housing curved beyond the slant 6 mounting surface with exposed bolt holes, so it could be used with the small block V8. The W200 did have the NP 435, shifter on the floor. Both slant 6 trucks had 3-speed shift on column. I remember all 3 bellhousings being the same. I had thought the flywheels were universal because the bellhousings were the same and the small block 318 and the slant 6 were both internally balanced. I remember the starters also being the same. If I'm wrong, OK.
It is possible I am wrong. Don't have a lot of experience with trucks. My 64 D-100 has a bell housing that would only fit the slant six. Trans is a A-833OD.
I have a flywheel and clutch on hand, that is used with the NP435, and it requires a larger bell housing then the one on my 64 D-100. The flywheel will not fit into the bell on my 64 truck.