i just took my '66 on a 1100 mile road trip from sc to ky while towing a car, then up to oh and back to sc. It's 95% stock mechanically, i've only removed the stock fan and added an hhr fan/shroud for better mileage, a little more power, and less noise at cruising speed. It has a 250, three speed on the column, and 3.73 gears. Still has the original points distributor- i tried hei and it didn't start or run any better and the modules either went bad or never were right straight out of the box. Underneath it has a cpp disc/spindle kit, d2600 air bags in place of the rear springs (i wanted it lowered but still have carrying capacity), and the rear shocks are relocated. The rear shock relocation required drilling a hole in the frame, one on each side but that could easily be welded up later on. Nothing else has been irreversible.
Would a v8 with overdrive worked any better? Not really, it held 70-75 mph up hills while towing and would cruise at 80. 85 mph is 3700 rpm so it's starting to run out of steam at illegal speeds, but pulls adequately even when towing until then. Taking the fan off eliminates the "screaming engine" noises at speed that everyone complains about. It got 12.6 mpg towing at 70-75 through lots of hills, which really isn't too terrible without having overdrive. It got 16 mpg empty at 60-65 mph, again not terrible.
My main complaint after the trip- wind noise from driving with the windows down at 70-75 mph will give you a headache after 10 hours on the road. A/c would fix that. I left the windows up on the return trip, had the kick panel vents open, and the passenger vent window cracked to let air from the kick panel vents out which worked great to keep the cab cool with less wind noise- no headache when i got home.
You can upgrade everything, but it's not really *necessary* to get them out and use and enjoy them. If its a fairly original truck then i'd say lightly modify it if you want but don't do anything irreversible, past that find one that's already modified if that's what you want.
