The gentleman that owns or runs
https://www.olsonsgaskets.com/ (pretty much the only place I can find oil pan gaskets for the GMC v6) needs a donor engine to reverse engineer the rear seal gasket.
I'm guessing this engine is likely the same internally as the 305 (for his purposes) with the exception of the pistons/cylinders?
You may see him pop into the classifieds.
He saved my bacon because he has gaskets for our trucks.