Pinching off the return line will give you a small increase in fuel pressure (in a properly working pump). It may not do anything for you. But really, fuel pumps are cheap. If you have the original, or you can't remember changing it, replace it. If you going to disconnect the exhaust, try disconnecting the catalytic converter. The converter is the most likely suspect to clog up the exhaust. Mufflers, and pipes rarely collapse, but it happens.
same problem I had your problem on my 76 Electra with 455 engine and quadrajet. I've replaced that wonderful emission system with an Edelbrock Performer 750cfm, and now there is no more issues. You're running a 27 year old car with dinosaur emmision system. The hear riser could not be opening, the egr valve may be clogged, the ported vacuum switches could be cracked, broken, or jsut faulty. Lots of little stuff to stop the big stuff.
