If in fact it is the rear main it must have been installed improperly. That is just too much
oil for a rear main. Re-check the valve covers and manifold. Also, check the
oil pressure sending unit and
oil filter.
If it is the rear main you may have other problems!
By the way if I remember correctly the rear seal on a 302 is a two piece rubber gasket and can be replaced with a tool that allows you to snake the gasket around the crank while everything is still in place. I tried it once and it worked out ok.
Clois