Even with a new set of gaskets, your bores are probably a bit egg-shaped by now and I'm sure it has some blow-by. It may run forever, but I wouldn't stress the thing out.
Exhaust may get you a little more power, chances are you won't notice it though. Stock exhaust tends to be of higher quality than much of the aftermarket stuff, so you'll probably be taking off a lifetime system to put on one that may last a couple of years.
Just run it, 300's were really never meant to be a performance motor.