Buy the gallon kit and plan for at least 3 coats. And frames are tough. You'll want it on a rotisserie or an engine hoist at one end to raise it high so you can get underneath.
I've painted one frame on a rotisserie which is very convenient, a frame I just did this summer I didn't remove any of the suspension or rear axle, so I put it on tall jack stands, sprayed the top and sides, and then hoisted one end up as high as I could with an engine hoist and painted the undersides until about midway down the frame, then moved the hoist to the other end of the frame.