my take
1) nothing wrong with 6V or 12V. I'd probably swap to 12V when it is convenient, might want to get it running with 6V as is for now.
2)Newer wheels will fit so long as they clear the brake drums, and the 8 lug on 6.5" circle was used for years and years later, lots of wheels for you to pick from and test fit. 19.5" wheels look and fit very nice if you can find them. Or if your multi- piece 17" wheels are in good shape, not rusted, you can still run them.
3)Reliable shop truck, keep it simple and keep the 6 cylinder, a 55 model 235 is already an improvement. Sell the 350 and auto.
4)Yep, look at what Mar-K has to offer.
