The 66 truck looks like a good deal and it's close enough that you should go take a look at it. It looks like it just needs some TLC.
I think the 51 Chevy is what you really want though. If you can't get a 49 Merc at least it's from the same era. Anything you could do to a Merc would look good on the Chevy too.
If the wiring is still good it can handle 12 volts. You just have to change the bulbs When you change the engine the starter, alt and everything on the engine will already be 12V. That just leaves the radio heater fan and instruments to deal with. The Radio is easy to replace with a modern unit. The Amp meter will work at about half the amps that a 6V system used. There are resistors that you can wire into the other 6V circuits to make them function on 12V or they sell batteries with a third terminal that is 6V so you can run 6 and 12 volts from the same battery.
Another option is to toss out the instruments and make a custom dash panel with new gauges or adapt some from another car. They didn't say there's no interior. It might just mean that it's in rough shape inside.
Here's my low buck interior tip of the month.

If it has the original seats J.C. Whitney sells a nice set of custom fit tuck and roll seat covers that are more like new upholstery. The price is $240. I used them on a couple of cars and was very happy with them. They have patterns for almost every old car made and your covers are made to order not taken from stock. They also sell matching tuck and roll material to make door panels so your interior doesn't have to be expensive.
I used a white set in a 64 Fairlane and made matching door panels. I couldn't believe how many people thought it was a big dollar interior. I used black in my 56 Packard because it's easier to keep clean.
Now that you're a senior CHR member you gotta buy a hot rod.