My son LOVES 40s. He came home from hospital in my 84 FJ40 LX and when we moved to America with it he helped me put it in the container in Australia with our house stuff and then he drove in it with me to Santa Fe from the customs house in Albuquerque when the container got here. (I'm Chamba here on Mud- the story is there- but I only seem to be able to log on to my older handle here for some reason) He has strong memories of it.
As for the truck, you have to think like a 12 year old boy: in the now. A shell of a truck isn't fun to him. A RUNNING truck is.
My son's Camaro has 48,000 original miles and I bought it from the original owner. He got to ride in it straight away, so all the labour we put in to it produces immediate results for him. The new suspension gave immediate results and he saw how rewarding his efforts were. The Dakota Dash was immediate, as were the Corbeau Z28 seats and Magnaflow exhaust.
I think a shell is great for an older kid, but I really think a running car is the bait you'll catch a 12 year old boy with. Build that awesome truck, but maybe buy a running one to reel him in. I would look at a Ranger or older Toyota Hilux that needs minor tinkering.