A simple low buck fix. Fab a spray bar for front top of the radiator core, simple pipe w/ small holes. Attach to a gallon or 2 water supply & divert your windshield washer pump or any 12v switch controlled pump. The gauge will drop in minutes.
Not at all familiar w/ your ECU set up. However some get whacky from radio interference. Easy enough to disconnect the hot to the radio or any com device in the truck.
I ran duals for a year on the K20. Black valley flashing screwed to the wheel well lip looked better than the numerous busted fiberglass fenders seen around here. I abandoned the duals after one winter of following the crown to the barrow ditches.
Ive done trailers, 55 chevies & K20 with build it yourself components from Superbrights. Devastatingly bright & very inexpensive.
https://www.superbrightleds.com/vehicle-lights
Welcome, Payson used to be my escape from the valley in summer. Hell’s Gate was my fav get away.
Likewise couldn't find cab mounts. Ended up using hockey pucks
Haven’t looked at a wiring diagram, but what you describe is common misswire in 55-57 chevys at the ignition switch, or a bad ceramic dropping resistor mounted on the firewall. I’m guessing you have an HEI distributor. My wager is on it or its wiring.
Many years ago I dug in & learned the zen of just how Quadrajets work and their common issues. I never considered jobbing out something that you could learn & do yourself. No dealing with getting screwed. No question about what was done & good insight to any remaining issue. It’s called a man...