Poor Idle, loss of power light

I just bought this reliant, did head gaskets and noticed nothing wrong. Did not have exhaust pipe and drove 200 + miles. I put in a 1 3/4 straight pipe and drove back I noticed exhaust pipe and manifold redding hot. Someone told me its because I didn't have cat converter, so I ordered one 2" diameter cat converter and a 2" resonator and I have the muffler. I haven't installed it because I want to change all intake mass, carb and throttle sensors but before I change them I want to make sure that the new exhaust system did the work. I drove 200 + miles without a problem the problem was welding a pipe from the manifold flange back, and that's what cause it to red. I'll keep you informed after I weld the new parts on.