81 firebird stalls at intersections and smells like exhaust

I have a 1981 pontiac firebird with a 400 chevy v8 small block engine. It has around 90,000 miles on it.

I bought the car about a month ago and it runs pretty well except when i am driving at slow speeds, or stopped at an intersection my car will stall and turn off. It always starts again but it is a pain in the butt. Also when i first bought the car it was extremely loud because it was running straight pipe and had no exhaust or mufflers, so i clamped some on. after that it was much quieter, but while driving there was a very strong smell of exhaust inside the car. I assumed it was becuase there was still a gap between the pipes and the clamps, and the exhaust was leaking through them. So i brought it down to the shop and welded them shut. There are still small gaps near the top were i couldnt get at with the welder, but i dont think its enough to leak that much. But the smell is still there and i am afraid that it could be dangerouse to ride in the car with the fumes still there. Please help if you know what the problem might be. Thanks.
johnster; If you have exhaust leaks, the oxygen sensor(s) may not be getting the correct information. If they are seeing too much 'air', they may be telling the computer to go rich to compensate. Have the exhaust system repaired PROPERLY. Clamping things together is not 'fixing'. You may be looking at PCM problems, as the '81 came with a 5.0 or a 5.7. The IAC (idle air control) controls idle speed. It may or may not be working properly due to the PCM or just needing cleaning. Any time things are 'basterdized' like this, you are just asking for problems. Sorry.
Loren
SW Washington
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