Backfires thru intake & won't accelerate?

Also changed the plenum intake gasket

sounds like the timing chain is worn out , I'm not sure if lack of O2 sensors and cats would cause that but it might . I'd say after 23 years that motor is beat .

This was common back in earlier days, the cam lobes would wear down and not open the valves fully and you would get a backfire through the intake, so this old it is highly possible, you would need to remove the valve covers and measure the lift of each valve, of course even if you only found one it would still need a cam, lifters and gear and chain set.

The motor itself only has 155xxx miles on it been very taken care of, the cats been off of it for well over a year now which eliminated the o2 sensors. We've also adjusted the timing and got it TDC but it's still sounds like it's sucking air from somewhere around the intake and we've done replaced the plenum gasket as well as sealed the gap where the manifold is a Little off and that fixed those leaks but it's coming from the backside of the motor only when it's idling up when still in park. I need my truck fixed and tired of throwing money into it

Timing is not set at TDC you need some advance . No wonder you're having problems . Sounds to me like putting money into that truck was one of your hobbies .