whats the difference...318 SB and 318 Wide block?

I think there might be an issue with the size of the torque convertor hub where it fit's into the crank......
Theres some funny stuff that goes on there, it is a year dependent tihng. Of for which I'm rusty on and will decline to comment on.

Pretty sure you can drop a LA crankshaft into a poly.
Yes you can, though if you reuse the old trans, you will have to be careful of exactly what was there and what is going in at the cranks flange, flex plate, torque converter fitting all togther. Read my statement above.
I believe the really early stuff had a different crank flange??

Year dependent.

I am not sure at all on the motor mount issue.