Relax, its just an old car. It will all work out in the end. Work on one issue at a time if you can. For the thermostat, make sure it is seating correctly in the housing, check your housing for bend or warp with a straight edge and replace as necessary. Make sure all remnants of the old gasket are cleaned off both surfaces. I normally don't use any sealant on these, just a good gasket. If it isn't shifting down quickly enough, is your kick down linkage properly adjusted, is it even there? Sometimes this gets left off on a swap but is very important to make your tranny function properly. As for the carb, a vacuum leak can cause what you describe with the new carb as well as what you said was going on with having to use excessive choke on a cold engine before. May not be at the carb itself. A bad vacuum advance diaphram on the distributor or any open or broken vacuum lines could cause a leak. Is the carb stock or properly sized for the engine, too big a carb can really kill an engine at low rpms. Vacuum leaks, timing, point gap are all good things to check. Pick one issue, tackle it with a clear head and post here for help.