I know this is an old thread so my warning my not be relevant and you may not have the same engine as I did. I had a 1986 Astro mini van with a 4.3L engine. I made a very nice looking grill cover out of nogahyde (or some tough material) that improved my mix mpg by about 1 1/2mpg and highway at a little over 2mpg (also with big passenger side mirror folded in, yours is pretty bulbous to). But, there’s always a But, it’s not worth it. It’s not that the engine heats up too much but the HEI electronics does. I fried two of them and they are expensive. The first time I thought it was just old. After the 2nd died the grill cover came off and no more problems. I talked to a mechanic for a national electric company and he said all Chevy HEI units go bad on V6s because of heat. They weren’t built tough enough, their location on the distributor has no air flow and it’s in a engine bay hot spot. Their fleet truck’s HEIs were constantly breaking down, multiple times, in the field that was costing them a fortune. Their solution was to install a flexible tube from under the bumper to the distributor for fresh air to cool them off. No more breakdowns. They began doing this to all their trucks.
The van eventually died & went to the junk yard. I got a 87 Dodge Daytona 2.5 4 cylinder turbo that’s been hot rodded. It can’t get good city or mix because it has ganged fuel injection and I’m not good enough to change the programming but others have and get 3mpg better in city. I’ve also installed a belly pan and other aero mods including lowering the car ½ an inch. Using air shocks to raise and lower (highway) your truck would dramatically improve it's drag coefficient.
Milage: City = 17, mix ~ 24 but at 55mph highway it shines, perfect conditions it’s 38-44 mpg depending on gas quality and distributor timing setting.
Not a mistake, measurements taken with the odometer and scan gauge and it does it every time.
Aerodynamic mods work! I started aero modding in 1977 with my 72 Pinto.
Stock Daytona drag coefficient was supposed to be about .33, this one could be as low as .28.

Good luck! Bob.