I like that Pontiac, and if you want something that you/your son can actually drive it would be by far the most reliable and practical of the bunch.
The Jeep....I don't get it, looks like a ton of work.
I sort of want you to buy the Spitfire just because I want to see how the swap was done, lol. IF it's not hacked up then swapping back shouldn't be too difficult; but I could see little details like wiring and cables (speedo specifically) being annoying.