Is exhaust back pressure the culprit here?

There are a couple of places on an A-body where the stock fuel line routing and exhaust come very close. The one that comes to mind for this problem is where the fuel line exits the fuel tank and runs over to the passenger side frame rail and then along the inside of the frame rail. "IF" your car uses the stock fuel route it is "possible" the new tail pipe is too close to the fuel line causing the fuel to boil. As a temporary check you may be able to wrap a bunch of aluminum foil around the fuel line to see if it lessens the problem. Thermo sleeve and an exhaust heat shield will eliminate the problem if that is the cause.