I put together a '54 GMC 1 ton flatbed with a 302 inline six from an M211, TH400, GV overdrive and 14 bolt with 4.10's 15 to 20 years ago. I do not put more than 5k miles per year on it, and not a lot of highway speeds. The GV I used needs no TPS or ECM input, not sure about newer units. I ruled...
I used SEM on the door panels, dash cover and all other plastic/vinyl trim on a '84 blazer. It held up ok, I was not gentle with the truck, would use it again.
I had the same issue when I bought a C&C then put a bed on it. If you narrow the cab/bed gap the rear tires won't center in the opening and look odd. I adopted the "learn to live with it" attitude.
I did mine with the dash out of the truck. Test fit and trim if necessary. Prep work is key, deal with any cracks, clean the old vinyl and get extra silicone. I placed mine upside down with some weights on it while it cured.
The original pump on my '05 rotted out around the return tube, the shaft on the first reman split trying to get into the driveway, the second reman is noisy but still hanging in there. I now have a third reman on the shelf ready to go.
I've had both 6.2NA's and 6.5TD's, I thought of them like "restrictor plate racing", plan your pass plus add 300 yards and use the downhills to your advantage. I still have have a '93 with mechanical injection and a 4L80E that goes ok, just can't be in a hurry.
Or....solve any air intake issues and use a stock engine mounted pump. My old '84 C-10 6.2 with dual tanks had pin holes in both sending units. Replacing the sending units solved the sucking air problem.
Have you considered a gear vendors or another brand OD unit? They can be pricey but, if there is nothing wrong with the th350 keep it. I use a GV OD behind a th400 in a 1-ton flatbed, I just needed to have the driveshaft shortened and minimal electrical wiring.