That would depend on several factors. I've always liked having the extended cab options and the newer "6-packs" definitly have more room. But to get the mose efficeincy, you would have to go pre-cat days. If you are happy with the room in single cab, then pretty much any 60s-70s would do. Maybe look at Fiberglass body panels to lighten them up a bit.
Basically, if you go pre-cat, not even Cali pays much attention to them these days. OBD-I systems give you more options than OBD-II (1996-newer). '88 and newer also may have roller blocks (if not, you can easily get one) which allows roller cams instead of flat tappet.
If you live in a DEQ area/state, I would say go Pre-76 on a half ton, not for sure when they put cats in 3/4 and 1 Tons (had a 77 that didn't have cats). This gives you the most options on timing and Fuel/Air to maximize power/efficency.
If you don't live in a DEQ area/state, I would go with an 88-95. This still allows you some play room even if DEQ and without DEQ, as long as everything looks right, they don't mess with you, no matter what you have done in the computer. I just like the room and handling on these models better. But emissions equipement for the swap will cost you extra.
I have a 2001 Silverado HD with the 6.0 liter, extended cab. Love the room, ride and handling, but other than gearing changes and low stall cat, maybe headers, I run up against EPA again. Not to mention retuning options are expensive (and I know people who were plaqued with problems after using programmers). Using Tunner Cat or similar, knowing what you are doing, you may avoid some of the problems, but still expensive for what you get for it.
I don't know about your state, but a lot of states, the vehicle has to meet standards of year of vehicle or block, whichever is newer. This kind of limits you to the 85-86 model TPI on pre-emissions vehicles, but no big deal. Unfortunately, I don't think you can put factory TPI on vortec heads, otherwise that would be a great combo.
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