body work over old paint?

65 year old paint is not a good base to work from. Strip it down to bare metal, remove all the rust you can & encapsulate what's left in the surface piholes. Coat it with epoxy primer. you can apply your body fillers right over the properly prepped epoxy primer. That will give you a much better base to work from & prevent rust much better.
The original paint was lacquer & is known for shrinking, cracking & crazing. Putting anything directly over it is asking for it to look bad.

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I agree. Engine heat, and hot summer days will cause the lacquer to expand at a much different rate than a new water based paint and clear. Blast (aluminum oxide or some material that won't generate heat and warp your panels) and prime. A good autobody supply should be able to give you some good advice, but I really liked a PPG DTM (direct to metal) epoxy. It's tough to get here in Canada because of VOC laws, but I think it's still available in the states. Give it a bit of a scuff and do your body work right over it. It's been a long time, but we also used a product called "Spritz" and "featherfill". They were a spray on fillers that went on over etch primers (after all the metal work and body fillers) that we would cut with 180 and 220 before adding high solids. The result was super straight panels. Because the industry changes quickly, I'd get advice from an autobody supply shop or a rep from PPG, sikkens, or something.