a good investment would be an infrared temp gun, allowing you to get actual temp reading, by pointing at various locations. it sounds like the block needs to be flushed, and verify the waterpump is the correct one, for the pulley. (or visa versa) the 180 stat sounds like a good idea, and drill a hole or two into the plate on it, maybe 1/8" to allow it to "burp". radiator sounds good, since you had it tested. you might pressure test the cap, just to ensure it's actually working. (new doesnt always mean it works)

getting hot enough to melt nearby items sounds like a fuel issue. are you sure the carb is set correctly? float levels okay, seats not leaking, etc? and finally, are you sure fan is sitting properly in the shroud? remove the belt, and spin the fan to check for free operation of the pump. no noises or resistance? just a couple of easy to check ideas. the temp gun, is money well spent, and you'll find tons of places to use it.