Will the block plate from a 250-6 work on a 302. I know the bellhousing is the same. If not is it neccessary?
No and in most cases, they are required. Depends on the torque converter clearance to the flex plate. Also starter location, to some extent. Have run several engines without them and some that "had to use them". Even have used a shim between the bell housing and trans case to get the proper clearance on the converter, but that's another story itself in racing applications.
I used the block/tranny spacer plate that came with my 250 andc-4 with my new 302. No problem, Used the same 250/c-4 transmission(had it rebuilt and upgraded). Just have to use a 157 tooth flywheel with the correct balance for the engine, 28oz up to 1982 and 50oz for 82 and up. bought my flywheel at the ZONE for 39.95. Hope this helps...jerry