1975 Chevy 3+3, 4-door, 3/4 Ton Truck

I was looking at this truck, its better than the GMC I looked at, I like it a bit more. however, are there any major concerns I should look at? like why the A/C doesn't work?
so, what is a 3+3 "6-pack." ?
a 3/4 ton or 2500 class correct? (its been a looong week....)
what is the 454 cu in (7.4 L) V8 fuel mileage? what can I do to make it better? an exhaust, and intake etc. etc...
my other question, is the low mileage a good or bad thing? I can see less wear and tear, but I can also see problems with sitting for awhile...

also, with this age of pickup, how hard and what is the cost to wire in a better sound system?