What is the purpose of the flex-plate on 350s with automatic trannys and can new crate motors use the old one even if they give a recommended part number?

The flex plate connects to the crankshaft and bolts to the torque converter. The flex plate also has a ring gear that meshes with the starter. If the flex plate is in good condition it can be re-used. Check the gear to make sure that none of the teeth are worn. If all teeth are still in good shape and there are no cracks in the flex plate it can be reused.

very commonly found in older Ford f250