Mopar Forums - LA 360 Idles great but dies in gear

I've been trying to chase down this problem for 2 weeks.
Had to move back in with parents so I scrambled to get the truck running. It drove 27 miles from my place to their place.

On the last leg of the drive I noticed a significant power loss. This was a really steep incline and at about +3000 ft elevation compared to the start of the drive.

It idles great and starts fine (at their elevation). I can let it warm up easily too. When I go to put the truck in gear though and give it gas the second it seems to begin taking on a load the engines sputters and dies. If I really gun it I seem to be able to overcome that loss.

LA 360 mated to a 727 torqueflite and NP205 transfer case

I recently did a few things to the engine.
Swapped on new Edlebrock EPS intake
Holley 4 barrel on it (it was what I had lying around but it had never been used) - has a spacer underneath, new gaskets on both sides.
New Kickdown cable and throttle linkage
All new vacuum hoses (fitting at the booster spins freely though - should it be tighter?)
new spark plugs
New Wires
New cap and rotor (electric not points)
new ballast

I have tested a couple things but I'm clearly no expert (This truck is almost twice as old as I am).
- Timing seems to be at about 5*BTDC at idle
- Vacuum at idle seems like it might be low at 15 in HG - but would that really kill it in gear? Does the elevation have that much of an impact? Is that low enough to indicate a leak somewhere? Or just worn out parts?

-Engine does have high miles and I know it has some old gas in it but I mixed in about 8 gallons of new pump gas and it was running great down in the flat lands near my place.

What would you do next? I know there are a lot of factors going into this one and I'm feeling a bit overwhelmed.

As far as I know it has a stock cam.

Really appreciate guidance on this one. Thank you.