I have a1990 grand prix - V6 it seesm to run okay but at 3800 rpm it will misfire and will not go about 3800 rpm. Amy suggestions?

I believe I have a leak in my heater core (5.0L engine). A few months ago I noticed water in my floorboard occasionally. Then my defroster and a/c vents began putting out a sorta fog that smelled like antifreeze whenever I tried to use heat. With cooler temperatures here, whenever I park the car and come back to it, there is a film of moisture on the inside of the windshield. There seems to be a small leak of antifreeze just rear of the passenger front wheelwell. I really dont know where the heater core is located on this vehicle. I spoke with a garage about this problem and they suggested it was the heater core and quoted me a price of $200-$250 to replace it. Local parts store has this heater core in stock for $21.88 and I understand it would take a professional 2 hours to replace it. My question is, instead of replacing it, can I use a radiator additive like Bars Leak that claims it stops leaks in radiators, heater cores, and lubes the water pump? Just need a little advice from someone that knows more about this stuff than I do. Thanks for any help :confused: