I am rebuilding a cylinder head with OEM parts. My question is does anyone know if there is a big difference between the the 420A valves and the 4G63 valves. I know they are longer but dont know what length they are or even if its possible to put the valves in.

I have never honestly rebuilt a any 4g63 head. But I have used Chevy valves in Ford heads. Its quite common in the hot rodder world. Valves are pretty universal as long as you are willing to make them fit properly. Seeing that I doubt that you have a full machine shop at your disposal, I'd stick to stock valves. Have you checked your guides and be willing to cut valves?