The flywheel won't both up from one piece (external balance) to 2 piece (internal balance) engines.
However, used to see plenty of issues with small block 400 engine being external balanced uses differant flex plate & damper than all other 2 piece main seal small blocks but fit like a glove!
Along with the motor mount might want to look the transmission mount over real good. Seen small gravel get jamed into a mount & cause a ground out. Seen rocks jammed between crossmembers & transmission before.
