1985 Monte Carlo

I have a 1985 Monte Carlo and I have put new plugs, Plugs wires, one of those Catatic Conver I punch a hole in it because my brother told me that might help , I check all the vaccum hose and I put a new rotor cap on it. When I give it gas on the interstate it back fires and it has got real hard to start. I drive it in second gear and it don't back fire as much I don't what more to do can any one help me.
Kohala55