[Gr1m]

first of all, if your leaking fluid theres a good chance that air is getting in your lines. second of all you havent replaced anything major or ported anything to notice any difference, you got rid of a air box and put a filter and you did a catback exhaust, none of which has to do with your coolant/tstat, and you said that it has been doing it since you got the car, and im guessing you tuned the PCM after you bought it so obviously that isnt the culprit either

...so you either effed up a gasket on your "3500 swap" or you ruined your thermostat when you decided to drill holes in it. when you replace your theromstat this time put some RTV sealant around the tstat housing where it meets with the LIM and see if the leaking still occurs. if that doesnt help then your leak is somewhere else and your definitely getting air pockets in your lines

dont get so defensive when someone is trying to help you.