That can be done without a full "off frame" restoration. Clean and paint the outside as needed, and there are all sorts of products that can be sprayed inside the frame rails to at least prevent further deterioration.
Agree with Andy. Here's what I did on my '66 TR4A solid axle where the body was fantastic but the underside showed some age and surface rust. I'm very satisfied 9 years later: I put the car up on jack stands, removed the tires and the front left and right control arms. I slid under the car for about 4 weeks off and on, wearing a breathing mask and mechanics coveralls I cleaned the entire underside with Marine Clean, a product from POR 15. It's a petroleum free product that can be a bit harsh on the skin, and not good to inhale, so I wore gloves and a mask. That process went on for a long time, as each night after work gobs of grease and grime fell onto sheets of newspaper I'd laid out under the car. I'd use an entire paper every day. I sprayed the cleaner into the frame holes and blew the residue out with compressed air. That was tough and of questionable effectiveness.
Then, using an electric sander, and even an angle grinder with a wire disc, I removed all visible rust from both the frame and the floor boards. Took quite some time.
I then applied the POR 15 Metal Ready, and followed that with a coat of POR15 which is advertised as: "POR-15® is a high-tech, high performance rust-preventive coating designed for application directly on rusted or seasoned metal surfaces. It dries to an incredible rock-hard, non-porous finish that won't chip, crack, or peel, and it prevents rust from recurring by protecting metal from further exposure to moisture."
The underside of my car looks terrific, and I've not seen any hint of rust. However, my car is a born and lived California car, now Arizona.
One last thing: I bought a tub of Waxoyl and sprayed that into the holes in the frame when I was done. What a chore that was. The Waxoyl sprayer is just nothing to rave about. I should've used the aerosol product.
I also rebuilt my engine with the transmission and block left in the car. Very happy there, too, but if you do that be sure to carefully guard against any debris falling into the crank journals.
I'd go "frame on" whenever you can, and then drive the car.