Dave, these kind of cars are the most fun to build, they are free form, not a color by numbers kind of car like a muscle car resto!

First thing I'd do is figure out what you want. Hot rod, custom, or just a modified driver? 50's or 60's style? 70's or later?

A stock '38 Plymouth Coupe is a good looking car, you won't have to do much to the outside unless YOU want to. Don't modify it just to modify it, a fairly stock exterior over a lowered modern chassis would be a good start. I'd go to a few shows and see what ideas you liked on other cars and see how you could apply it to your car.

Really want it chopped? It really isn't that difficult to do, there's a really good book by Tex Smith about chopping cars. You're already cutting and welding patches, so you can do it too.

You might just focus on the suspension, steering, brakes, and wheel/tire combo for now, and let the rest develope as you finish up those projects. You will need to do them anyways, and this will give you time to figure out exactly what you want.

A carbed v8 and automatic tranny is about the simplest swap into anything, and you'll need to put that in there to figure out where other things like steering, etc will be going.

As far as gas tanks and seats, this stuff isn't rocket science, go to a junkyard with a tape measure and have at it!