Fire trucks carry 1000 gal H20 @ 8.34= 8340 lbs. Yours may have had a larger tank than that!!! Maybe it's built special with heavy duty frame or something? 20K miles should run strong. Something big does seem to be wrong. Is it winding up OK? while climbing/hauling or is it bogging down? Ebrake stuck on?
Something goofy like that could be happening. Two speed rear end? 5 speed trans? I'm no expert on hauling with big trucks but do you keep it wound to highish RPM using proper gear changes? Could it be you? or are you experienced?
It is made to haul and it should do a good job.
I would absolutely check all brake shoes and parts at each wheel if you have never looked.
Been reading your past posts. BBV6 mentioned 5 deg BTDC, you said 10. I think 5 means 5. If it won't run there, find out why, don't use 10.
Snazzypig questioned your jets, gasket arrangement, vacuum, etc. I think you need a new stock carb. I think you have tinkered it into a mish mosh. Start over, do it right, don't screw with Mother Nature. These motors are not made to customize and tweek around on. They are designed by engineers who perfected it already. It ain't no SBC. Leave the jets alone. Assemble carb carefully, ALL NEW parts, setting level and drop as necessary. You are only 450 ft above sea level unless you are on some big mountain. So that should not be the problem.
I am not slamming you, just cutting to the chase. Hope you understand. Don't want to blog it to death, want to fix it. Making sure you are out of the past box and into brainstorming. You have added many new parts. Double check your work and your points gap. Tune it all TO SPEC, not someone's BS ideas. You can play with timing a little. It's not what it sounds like, it's what it runs like. I think you may have said it "pops" at 5 deg? Make sure that is Before TDC and not after. If it backfires at spec then it's vacuum leak....big time. Even an exhaust leak will do that. Analyze some more and see what you get. Please state if it's a carb backfire (pop) or a tail pipe backfire....at 5 BTDC. We'll get there...me, Snazzy, BBV6, TJ and anyone else who can try.