Based on the pic you posted, I've changed my mind about this car and wanted to make a few points. I may not have grasped what you were asking at first but based on the pic, it needs a total restoration.
Like I've said many times before in these types of threads, it's likely that every piece of rubber is shot on that car which includes weatherstripping, bushings, window gaskets etc. I would say that if you plan to keep this car for any length of time you should strip it and do the body first. Brakes are a good start but trust me, it will snowball from there once you see how things look that have not been serviced for decades. Might want to consider that the exterior sheetmetal is only part of the question.
It would also be prudent to make sure you know the extent of the rust that can not be seen. If there is rot in the quarters then there's a good chance things like the trunk extensions, rocker panels and wheel houses are rusty. Not saying definitively yes but it's not a stretch by any means.
Also, if you're putting a big block in then you'd really want to consider putting frame connectors in to prevent body twist with the added torque. Not that a 383 is a monster but it should have more torque than your average small block, not to mention adding weight to the front end. So considering the idea of welding on the subframes, ensure they are straight before doing any other work. Wouldn't want to weld in the connectors and find that you need to do some more extensive work than you originally planned.
My Duster is from Southern California and never saw road salt. I was willing to pay a premium for a car from the west coast so I didn't have to deal with any body/structural work and could concentrate on performance. Regardless of how solid the body is, I still spent the last year redoing almost everything - front and rear end, brakes, suspension, interior and on and on.
Here's the link to see what the underside of a rust free car looks like.
Just some things to consider before you start on this car in it's current state. Rust is the enemy, I think you'd want be 100% certain that it's only what you can see.