Samuraiox

I have a 1987 Chevy g20 Van. It has about 113,000 miles on it right now and I just upgraded all the interior from a 95 g20. It has the 5.7 and I've put 10k on it since I bought it earlier this year and have had virtually no problem...

...until a few weeks back. I've had a full reserve for antifreeze and it was all laying on the ground awhile back. I figured it'd be the waterpump and just took it to my mechanic to let him deal with the hassle. He called me back and told me the intake gasket was leaking it. So we put in some stop leak and that seems to be holding for the present moment.

I found a Vortec 350 and it just got dropped off the other night. The kid who I bought it from put an aftermarket intake but had it setup for a carb and fuel pump. I've just got to get a stop plate, no biggie. What I am wondering about is the Intake Manifold. Is there a certain one that'd be worth it to get the best gas mileage for my big G20? I also saw that there is an upgrade from the tbi? What is the best case scenario for this engine dilemma I'm having?

Also planning on going with electric fans and K&N filter (with tophat). Are there any other upgrades I could do to better the gas mileage? Would a tbi spacer be an improvement? Since the van is 87, there really wouldn't be a chip for any of that, would there?