So this is the '91 G20 that is in your profile, right?
I love those older GM full-size vans! About 18 years ago, I modified a '79 short wheel base van. By the time I was done, it moved out pretty nicely.
The exhaust is the place to start. One can get full-length headers that are specific for this chassis.
There is plenty of room underneath for a full dual exhaust. I do not know of anyone who makes an exhaust kit for it, but a custom-made one is super-easy.
I made one set myself. The full length headers place the collectors low enough that one can just run a straight pipe underneath the transmission support. Make sure it ends about 3/4 of the way down the floor. Then install 2 mufflers followed by a 45 degree bend. The tailpipes are fine exiting just in front of the rear wheels.
I had an exhaust shop make up a custom dual exhaust for me the second time. They made some nice over-the-axle tailpipes that gave me exhaust tips at the rear bumper.
Just put in a balance pipe between both sides of the exhaust, just behind the trans. It will reduce noise and increase torque a bit.
As for the intake... you should have TBI on it. There is nothing in the way of a better air filter set-up that fits under the hood. But I was able to add a second air cleaner snorkel to the stock air box using ABS plastic tubing for a home-made high-flow cold air set up. A K&N filter helps too.
Next, toss the crankshaft-driven radiator fan. Install an electric fan with a thermal switch.
BTW, 99.9% of the time, there is enough airflow through the rad to keep things cool. All the stock fan does is suck up power.
As for the TBI set up and cam, heads etc, as noted above, use mods intended for the '91 5.7L pick up trucks.