I don’t know of any pumps that could be retrofitted to your original tank.
My gas cap is not vented but the tank is vented with a Tanks Inc roll over valve.
I think most people mount their external pumps to the frame rail.
It looks like the 216 intake manifold studs are on 2 11/16 center to center. The Sniper Throttle base plate bosses are on 2 3/4 centers. You may not need an adapter???
As far as a return, could you drill a hole next to the sender and install a bulkhead fitting for a return?
