[HO455]

I fought a similar issue on my mom's GTO years ago. It ended up being a bad fuel hose. It had a fuel filter installed just before the carb with two short pieces of hose. The one closest to the carb had an internal flaw that had formed a flapper inside. The car ran fine until you got on it. Then at the top of first or midway through second gear it would just fall on its face and die. After a couple of minutes it would fire up and run fine. After checking all the usual suspects. I took it out and got it to shutdown. I coasted off the road and pulled the carb top and found the float bowl dry. A quick inspection showed the float, needle and seat all looked fine. While I was standing there with the engine shutoff scratching my head fuel started flowing into the bowl and I could hear the sound of liquid spraying come from the filter. I loosened the clamp on the filter and fuel sprayed out. After inspecting the hose I could see it was partially blocked. Replaced that 2" piece of hose and all was as it should be. The flapper inside the hose allowed enough fuel to run the engine until the fuel pump began pumping more as the RPMs increased then volume or flow rate or both shut the flapper blocking the fuel to the carb. Once the pump quit pumping it took a minute or two for the pressure holding the flapper closed to bleed off and open the flapper.

I would take your truck and get on it and when it falls on its face shut it off and coast off the road so you can pull the top off the carb see if the float bowl is empty. That way you will know if it is a fuel problem or something else.

If the carb checks out put a timing light on it and watch the flashes while you rev it up to see if you lose spark.