Figured I'd throw in my two cents regarding the intake conversation. The factory intakes on these trucks flow plenty for these engines and have excellent intake air temps due to being ducted to the fender, as mentioned. I'd advise against wasting money trying to improve something that already works great.
Granted both of my trucks are Vortecs so those with TBIs may have a different experience, but I put a K&N intake on my red truck and ended up taking it back off in favor of the factory intake. The factory setup gets consistently lower IATs, and has zero chance of fouling your MAF sensor with filter oil like the K&Ns and other oiled gauze filters are prone to do.
Stick with the factory intake and paper filters if you want the best option IMO. The only aftermarket intake available for these trucks that looks like it might be as good as the factory one is the Volant, due to the sealed box design that uses the factory fender hole. If you absolutely need an aftermarket intake for looks or whatever, that's the only one I'd consider spending money on.
Not sure where you got your information. The factory Vortec intake is very restrictive to flow. I consistently datalog 305 and 350 trucks dropping 10 KPA MAP pressure at WOT in the 4,000+ rpm range. That is over 3" of vacuum.
My old 350 TBI powered G-van also had an air inlet system very similar to a TBI truck and it would drop off almost 15 KPA at 4,200 rpm and that was with a K&N filter. The stock housing was highly restrictive to flow especially with the collar that raises the air cleaner and shrouds the injectors.
This is especially true once you open up the exhaust.
Best bang for the buck TBI fuel mileage and low-midrange power modifications I found were small 1 1/2 or 1 5/8" diameter long tube headers into dual 2.5" pipes into a 2.5-3" merge Y into a high flowing 3" exhaust with a straightflow muffler followed by ECM chip work. I also found running 13-14 psi fuel pressure to help atomization and torque which keeps you out of the throttle. Finally I also ran 25% underdrive pulleys with no ill effects. I had a stock 8.75:1 1-ton 350 TBI crate engine in my old 1983 G20 van. Backed to a 700r4 and 3.08 gears. With 1.6 full roller rockers, doug thorley tri-ys, merge Y, high flow cat, 3" single in/dual out magnaflow, edelbrock 3704 intake, open center TBI spacer with a 454 TBI unit and 454 air cleaner assembly it was a completely different engine. I would average 14 mpg city and 20 mpg highway with lean cruise enabled at 16:1 air/fuel ratio on level road at 70 mph. At light throttle, steady cruising speeds I had lean cruise coming on as low as 30 mph (emissions dyno tests were done at 15 and 25 mph)
