Gidday gents,
Hope you don't mind an opinion from down under.
I ran the standard carb 3.5 for five years in an off road only RR based buggy and through it all, four hours with no radiator (some idiot put the fan through it..... me) and hitting 4500 revs with the oil filter sitting on the ground, it kept going...
But, I have just swapped in a 90,000 mile serpentine 3.9 into the truck and what a huge difference. It runs a long snorkel which may be causing a slight lag when the accelerator is punched, but in every other respect it is head and shoulders above the 3.5. Lots of oomph and power (though in a light ruck) and runs on any angle (don't ask how I know).
Another solution that is more common in New Zealand and Australia is the rover based Leyland P76 4.4l motor. Basically a 3.9 with a longer stroke so it is "square" i.e. revs and torque. I have seen them with the standard twin cabs and with single Holleys or similar. Mine runs a single carb of a Holden 253cu V8 with petrol and LPG.
Go for the 3.9 if you can afford it, it's already been said anything mechanical will go wrong if it's not looked after, not coz it's a 3.9.