Your headers are way too big for your engine but if that is what you have then use them. You could've used 1 3/4 inch headers on that engine and been fine. You don't even need a 3 inch exhaust with that engine, 2 1/2 will work just fine, even 2 inch would've worked just fine. If you have the room and the parts on hand then go ahead and build whatever fits and works but it is going to be a lot bigger, heavier and louder than what it could've been. I'm in the process of ripping the 3 inch exhaust out of my '65 Coronet and replacing it with 2 1/2 inch since I'm tired of all the extra weight, space and noise. My engine is 600 hp and it doesn't need a full 3 inch exhaust system. Bigger isn't always better.........