ok - surprised no replies yet, so here's my perspective. I have a '70 C60 with the 366, 4spd and 2spd rear. While I thoroughly enjoy driving it, it is quite slow and tops out at around 52 mph empty. Fuel mileage really isn't worth calculating as there aren't many fuel stations that I pass by. The original brake set up works - certainly isn't like today's brake systems, but they do work. When I bought my truck I had planned to repower it with a Detroit Diesel, update the brake system to something more modern and basically feed my hot rod desires. However, after going through the truck, (it was all original ex-firetruck) I decided to pay homage to American History and leave it original and just enjoy the simplicity of it and the way things were in my childhood days.
It is my go to vehicle when I just want to drive and not when I have to drive.
I know that doesn't even come close to providing any answers to your thoughts, but maybe it'll get some conversations started.