1995 Chevy Caprice Lack of power and muffler overheating

Hello. My car recently been giving me problems. I have replaced my alternator, spark plugs and wires and fuel filter which was partially clogged. Upon acceleration it hesitates and sputters and low in power. It stops sputtering once I reach over 50mph with steady throttle with still no real power out of the LT1. Also my muffler got so hot that it had heated up under the rear seat and began to burn the insulation under the seat. I figured that the muffler got clogged somehow but theres a hole at the bottom. Need help