While I have the gas tank out no build sheet but a pencil size hole, I plan on freshing up the frame and it would be a fine time to change the differential fluid . Question is can I change out the fluid even if the car will not be driven for the next few months. Any suggestions on frame paint?. Some surface rust in spots Thanks. Big Al

Do you have access to a compressor and spray gun? I have found PPG DP90LF epoxy sealer/primer to flow out and cover better than any other epoxy I've tried to date. Epoxy is the sealer of choice for rust containment or when painting over older painted surfaces. The PPG DP can be sprayed as a single coat sealer/top coat if a) it will not be exposed to direct sun b) you use the hardener. Its a very nice black satin /suede finish...perfect for frames. If it gets a lot of sun it will get "chalky". It is designed as an undercoat but many shops use it as a single product solution for frames. It works on old paint just fine. If you have rust remove as much as is reasonable, treat with a liquid ruts converter and seal.
There are plenty of other epoxy sealers but none lay down like DP and have that perfect sheen. And yes you can topcoat it. I just shot my Model A dragster frame with DP and then top coated with a suede black urethane paint....the frame will be exposed so I needed the paint to fight off the ultraviolet. The axle and rear suspension are only covered in DP90LF.
And that is my .02