[Pontiac1976]

Ok not sure you knew what the vent looks like. But if you look @ the Diff 12 inch rubber tube on right side axel tube at the end is the vent tube check valve ( white or black Plastic).
The same vent tube check valve is on the trans ( white or black Plastic). ( lets gas vaper out keep oil in)
Is this the same old trans pan or the new one? that has the gap.

As for the old steel pan is it pretty straight the seal lip? And does the steel pan have a little ridge on the out or in side dead center all the way around the pan where the hole bolts go.
On the inside or out side lip use a flat punch or vary dull chisel to tap them back out the 2mm to 4mm depth this help seal the gasket & helps the bolts put a little more pressure on the flange. If they been over tighten they maybe flatten out to much on some or all at the area's around the flange.