[Lost in the 60's]

Looks to be a later model replacement engine with the center bolt valve covers...LT1, I think ? That could explain the need for an electric pump. The engine may not have been assembled for a mechanical pump ?
For as much as the engine bay seems to have been messed with, it is amazing all the paperwork was still in the glove box !!
Hope you replaced the shift shaft seal on the trans too, they leak when the vehicle sits and fluid drains back from the converter. A slow leak will migrate along the pan rail and make it appear it is a pan gasket leaking.