I dunno, the heater motor is easily replaced with a 12v, the one I used on my 51 costs a whopping $31, new from RockAuto. Since the insulation was failing on the original 6v motor this was a no brainer.
Wiper motor, you can buy a 10A 12v to 6v converter for $22 from Amazon, that takes care of the wiper if you have electric wipers, otherwise not an issue.
The only gauge that is voltage sensitive is the fuel gauge and that is only the later, non thermal types. Not sure it applies to the OP's 49 truck or not. But if your fuel sender has more than 1 wire then it's not affected. You can use a runtz resistor or just feed it from the voltage converter powering the wipers.
Swap the ammeter wires if going to negative ground.
Bulbs
That's it. Well you do have to swap in an alternator. Not hard to do, not expensive and no fancy brackets needed. In my case the 51 had 12v battery cables on it when I bought it, still was a 6v system. Lot's of other hackery the OP's mechanic did.
If you have a ride that has vacuum wipers, no heater and the thermostatic fuel gauge then all you have to swap are the bulbs. Almost so easy even a caveman could do it,
I had a guy stop by with a 51 Chevy that could have been my Plymouth's brother, black exterior, grey interior, coupes. Anyway, he wanted me to convert it to 12v because his headlights were too dim. After inspecting the setup turns out it had 12v headlights in a 6v system, No kidding they were dim, lol. I gave him my old 6v headlights and once swapped out he was happy. Saved him a few dollars.
I don't really have a dog in this hunt. But I do not like seeing people bent over at the cash register wanting to do something that isn't that expensive to do.