It seems you have it in hand. If you can get the hood and front and rear ends from bandit without buying the body that would be the way to go. I would much rather chop on a styrene body than a resin one if I have a choice. After some Googling and Wiki-ing you shouldn't have much trouble using the 69 body. There was a change to the Cutlass line that started in 70 that would have made the job slightly more difficult since the convertible switched form being based on the sports roof body like the 69 kit to being based on the supreme body which was shared with the longer Grand Prix and Monte Carlo.
You can get a very shiny chrome-like appearance out of it. Once it's on you can polish it with wax just like paint and bring it to a pretty high gloss IF you got it smooth enough when putting it down. If you have some it might be worth it to just try it on an area of a body to see if it's a road you want to go down.
The making old maps into decals is cool idea, you could even find the sections where they drew the feared sea monsters in different areas of the ocean. That would also help with the weathered, yellowed paper patina.
That frame is no longer available in that form and is needed to be able to build the AMT Kenworth W900A 925 kit. If you have or can find some more of the original 925 parts you can rebuild a somewhat rare and pricey kit.
You might try using a finer brush to be able to make thinner points on the palm fronds and things. Also, you might need to thin the paint some to keep it from chunking up. I have tried to use those hobby acrylic paints and after you paint for a bit the brush begins to gob up with half dried paint.
I remember that because I almost bought it when it first came out and the guy at the hobby shop told me about the missing fifth wheel and I didn't get it. I just bought the wrecker kit above and it does have both the wrecker body and the sleeper and fifth wheel but I don't know about other versions of the wrecker.