3.42's blow, unless all you want is help with fuel mileage. Never fear the gear, 80% of those year trucks have 3.73's anyway. Gears and a stall is what make these things move out.
Skip the shift kit and just buy a corvette servo, it's a better upgrade to the ticking time bomb tranny they call a 4L60e. Head swap should depend on what comes on them stock. 706 heads are decent but if it already comes with 243 heads, leave it be as they are already good. Otherwise it's kinda wasted cash unless you are making huge horsepower. Stock rockers are good too, just get the correct pushrods for your cam swap.
Also on a side note, that 2500 heavy chevy will be a tank and will have the stomach of a tank.