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Hello, I also have a 1991 Chevy Pickup with a rebuilt 350, 700R4, and 4x4. Is yours two or four wheel drive? You are already doing pretty good at 25 MPG, mine doesn’t get anywhere near that. Could you provide us some more details on what exactly you have done to the intake and exhaust and what you have removed as a part of the truck’s diet? If you haven’t already, routine maintenance is a good place to start, changing fluids and filters helps with efficiency and longevity.
Some low hanging fruit would be aerodynamics. Attacking the truck bed will be the easiest for the best return, to start you can do a bedcover. I don’t know what your budget it, but you can find cheap used bedcovers or even make a half bed cover out of one sheet of osb, just put it on the back half of the bed.
If yours has the recessed bucket sealed beam lights like mine, I would suggest making a heat formed lexan cover (or swapping for the non-recessed front clip). A front air dam will help as well, think of lawn edging below your front bumper to send the air around your truck, not under. You don’t need to go super low with it, just low enough to be below most of your underbody. You can also consider a partial grill block, especially during winter; just keep an eye on your temperatures (and have a way to remove it if needed).
Another big aid would be removing the engine driven fan and replacing it with a temperature controlled electric fan(s). You can get your fans from a junkyard.
This may be beyond where you want to go, but my truck had many 700R4 issues and is destined for a manual swap. I have a NV4500 waiting to go in.
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