eye, make sure you get the mild (aka RV) grind cam. Definite yes on the vacuum gauge - we get more than 1 mpg better by watching the gauge. Go with replacement manifolds (I got mine on eBay pretty cheap - and they look a TON better than the original).
Our rig was made into a true dual setup with minimalist mufflers when we got it. It was so noisy in the back, the poor kid had to wear hearing protection. We added a crossover pipe (made a noise difference but made a power/mpg difference, too) and then bought 2 Caddy mufflers from a junkyard and put them on in place - no power difference at all, but a definite noise difference. The kid can sit in the back without hearing protection now - she even fell asleep on the way back from NC.
I'm pretty sure ours is 2" diameter exhaust pipe. Goes up to 2.25" for the crossover, then back down to 2" behind. Which actually adds in a nice resonator pre-muffler.
What carb is on there now? When's the last time it was rebuilt/tuned? You'll see a difference if you get that dialed in. Also, take off the intake manifold and clean the heck out of it. Exhaust usually plugs up the ports, so cleaning it out will show a difference, too.
Since you're going to have the engine out anyway, clean out the coolant passages as much as possible, too.
