If your going to do both then I would suggest painting the engine bay and underside while the drive train is being rebuilt then you can go back to the body and interior later
This is what I did on my Barracuda. Got a pretty close match to the existing paint and did the engine bay. That'll save a lot of work and hassle later. Did all the cutting and welding on the rest of the car, touched up a few spots, finished off the interior and hit the track. It's a good 10 foot paint job that I'll eventually get to. In the meantime I'm having fun in the car.