My Cobra has a 390 in it and I'm very happy with it. Plenty of power, completely different sound than a small block (it's an FE, after all).
I purchased it from another Cobra owner that wanted to upgrade to a max built 427 side-oiler. That was a year ago and he still hasn't gotten his running due to problems with the Hillborn Fuel Injection system on it. Mine, with his old 390 in it, is running fine, sounding good, and getting miles on it.
I recently had a chance to drive a friends Superformance with a stroked 351 crate motor in it (392 cu in, 430 hp). The sound was completely different (I like mine better), power was about the same if I kept the revs up. His was a blast to drive and so is mine. The FE has more torque earlier but it's really hard to quantify since these cars are so light. 400+ hp in a light car is lots of fun. You won't be sorry with a 390.
Take a look at this website for info on building a killer 390 ........
www.webrodder.com and enter "FE" in the search. Look at the articles on the Cobra Venom build and the FE Ford - Hot Rod - Part 9 section for an example of what you can do with a good 390 block (431 cu in, 500 lb ft, 457 peak HP).
$200 isn't bad to pay for a non-runner but you'll have to tear it down and have a competent shop check the block, crank, heads, etc. You can buy a 428 crank that will stroke it to 410 cu in. Basically, a drop-in process if done by a competent FE builder. Numerous good ones on this site do that work (Keith Craft, Gessford, FE Specialists to mention a few) and back it up. Don't scrimp or it will cost you in the end. Do some research, buy some books, read the threads on this site.
Paul