Header/exhaust leaks tend to sound alot like lifter tick - take a pen and tap the end on a desk or table. Not really heavy. Piston slap tends to sound more like a knock, deeper sound.
Header leak and piston slap are two pretty different sounds. Best bet would be to get your hands on a stethoscope, the kind with the metal rod attachment. You can carefully run the tube along the header area and you'll hear the leaking area. If not, then attach the metal rod to the stetho and...
Don't worry about 'guys saying what a way to ruin a cuda'. Everyone has an opinion, but with your car it's YOUR opinion that matters most. Hell, paint the thing 3 shades of pink and make it a 4X4, if that's what you want then enjoy it. The whole reason we buy these cars is to have what we want...
You didn't mention price - if you're getting it for $500, then it sounds like a fantastic deal, $30K, not so much. Sounds like you're not into total restorations or trailer queens, so you can accept a car that's been modified - at a reasonable price. AS long as you're not looking to restore down...
How long does this last for? Does the engine show any signs of overheating? IS your coolant level ok? When the engine is cold, you are not getting optimum fuel burn, so you're probably just seeing incomplete ignition. If there are no signs of overheat, no coolant loss, or any other problem...
I see you've got a post on cuda-challenger.com, the pic you got there is the same info as mine - 3 wires only, same colors as described. Does your blower not work?