The changes you recently made - modifications to the stock exhaust - would not have any appreciable effect on idle speed. Definitely not to the degree you decribe.
Above normal idle is often associated with air leakage - specifically downstream of the throttle plates. But again, even a massive air leak wouldn't cause a idle speed increase up to the 2-3K range.
If it's a carbureted engine, I'd look at:
1. External problems (e.g. twisted/kinked throttle cable, bent or jammed linkage components, maybe even recheck that the throttle return spring is hooked up and in the correct place). To eliminate anything on the outside of the carb as the problem, disco the throttle cable and spring. Then with the engine running, use your hand to regulate engine speed. If rotating the throttle shaft to fully closed (CCW) doesn't bring the engine speed down you'll have to look inside.
2. Internal - with the engine off and air cleaner removed, open the choke butterfly and look down into the carb throat. The throttle plates should be fully closed. Rotate the throttle shaft open (CW) and observe that they open - and then snap shut by the return spring when you release the linkage.
I don't think a stuck choke would cause 2500 rpm but make sure the butterfly is wide open when the engine is warm.
If you have a stock Rochester Q-jet, pull it and look at it fro the underside. Any fuel dripping out? There are many reports of cracked float bowls that allow an unregulated supply of gas into the manifold.
I am certainly no expert on Q-jet problems but I'm sure someone can offer more help.
Maybe check also if your throttle cable/return spring is coming in contact with anything...
