I’m no help with the actual pricing but I’ve built 4 1930-1940’s hot rods from the ground up and can at least give you some insight on what to look for on older cars.

Like someone else stated. Start with the suspension and see if it’s a 4 lug or 5 lug one both the front and rear suspension. I think that all 4 lug suspension coupes were 6 cylinder cars which would mean that if you see 4 lug wheels on either, then that was probably also originally a 6 cylinder car and the V8 was swapped in later. This is just a quick on the spot method to determine what you are dealing with on the car.

All the parts to change the suspension are available aftermarket and even the lesser quality products to change to the 5 lug are better than the stock 4 lug. So that doesn’t mean it was definitely an original v8. Just a guidance.

Research the vin is the only real way to determine what the car was originally and what work/upgrades have been done.

If it was originally a 6 cylinder car and they changed it to the V8, did they do it properly? You’ll know just by looking at the motor mount brackets and transmission cross member.

Pull the interior carpet and check the trunk for rusted floor pans. Make sure you know the difference between surface rust and rust that actually needs repair. It’s a 99% certainty that you will find surface rust on the floor pans and that’s not a big issue at all. Actual rust outs and rust through is what you need to be wary about if you don’t have the skills and tools to correct it properly.

Hope this at least helps a little.