Tom , it may be best to have the cab sitting on the frame . This way you can check your distributer to firewall clearance in a real world situation.You can gently drop on the cab after the engine and mounts are mocked up to ensure all is well before finish welding or bolting in the various crossmembers. You also have to make sure the tranny clears the cab floor or firewall and that the engine downward angle is not so severe it upsets the U-joint angles.You can do it by measuring for sure, but the mock up may save time money and scratched paint in the long run.