I'm sorry, I'm just not sure what you are trying to figure out
Everything about your crate engine has already been mentioned several times in this thread. Since your crate engine was manufactured to be a direct replacement engine for various models of cars/trucks, it seems it has the provisions on BOTH the block AND the oil pan to allow for either a passenger side OR driver side oil dipstick. Whatever side you want to install the dipstick on, then the other side needs to have the plug installed in either/or/both the block and/or oil pan. Since the motor has a 2 piece rear main seal, then it would be similar to(but not exactly as) the 85 & older generation 1 small block Chevy engine. It isn't an engine that would have came from the factory installed in a production vehicle so it seems it is it's own 'breed'.
I'm not trying to sound rude, but I seriously don't know exactly what you are trying to figure out.
Do you want to know what year the engine is? It would be considered an 85 & older engine if it has the 2 piece rear main seal(which it sounds like it does/should) BUT that doesn't mean it will share the same oil pan that came on an engine factory installed in a production vehicle that is a 1985 or older vehicle. It is it's own design to allow for either/or oil dipstick location.
Can't you just use the original(to the crate engine) oil pan(with both provisions for either/or side dipstick) and move the plug in the block to the opposite side of where you want the dipstick to be? Or don't you have the oil pan that came originally on the crate engine(with the provisions for the dipstick on BOTH sides)?