Hei has a module or coil (don't recall the name) but it acts the same way when going bad. had to replace mine in my HEI but honestly don't remember the symptoms it had.. the coil or module is under the hei cap, should be black with I think two wires pluged into it. I would check all filters, make sure fuel pump had proper pressure (5-7Lbs), check carb to make sure you are getting a good steady stream of gas when throttle is applied, and if all checks out ok I would replace that module under the cap. I really doubt this is fuel related. It runs fine until warmed up, then starts cutting out, clogged filter wouldn't do that and bad pump wouldn't do that only when warmed up. sounds like heat sink in the coil under your cap. They work fine cold, and as they heat up they lose the ability to deliver the 12volts needed to run the engine and begin to cut out.