I wouldn't go any earlier than a '95 engine, if I could help it. P38 interim blocks started in '94, but in the first few months the cranks were fitted with the small bearing caps.

Oil galleries were kept small until the upgraded 38a engines came out. - can't remember the year though.

At least with a '95 and up, you get extra reinforced block, crank driven oil pump, big caps, ability to cross-bolt, thrust bearing/washer on cam, 10 bolt heads.