After A/C Add On Engine Overheats Really Bad

I just installed an a/c only unit on my 69 340 Swinger. It's a Nostalgia Air under dash unit. It puts out cold air but the engine immediately overheats to the point of the temp gauge pegging past hot when cruising or sitting at a stop light. The water pump is new, the heater core is new, I have a stainless steel flex fan that really doesn't flex that much, a 3 core radiator, 160 degree thermostat, and the timing is about 8 degrees BTDC. After I shut the car off, it won't start. I have backed the timing off and when it does start, it backfires thru the carb. If I move it up, it runs extremely rough. It ran great before the a/c install. Oh and the 340 is bone stock. I'm really tired of messing with it and I just don't want to touch it anymore, and everybody I've talked to don't have a clue. Any ideas before I yank it off and complain that I wasted hundreds of dollars??

Thanks