I think the 51 Chevy is your best choice for a hot rod. Changing it over to 12 volt would'nt be much of a problem. Changing the engine to a Chevy V8 would be a little more work. It has a torque tube driveshaft. (The rear end and trans are connected with a tube. The driveshaft is inside the tube.) So you have to plan on changing the rear end too.

I like the 4 door 65 Chevy. If it was a 2 door I'd be on the phone instead of the computer. It would be a good second car for when you don't want to drive the hot rod. It's a straight six not a V6. A friend and I pulled a 6 and put a 350 in one of these one Sat. morning and burned off the tires in the afternoon. It took us three hours from start to finish. It's a bolt in swap. The interior is still good and The body looks pretty good so with a paint job it would make a great sleeper type of hot rod.

The Pontiac is too nice a car. I don't think I could tear apart a car like that to build a rod. I'd probably keep it as an antique. Maybe I could do it for a customer but not for myself. But that's just me. I think Pontiacs had the torque tube driveshaft too.

The Merc would be my last choice. But that's only because I don't care for the design. That plus it would take your whole $5000 budget.