I would be really surprised if heads would make any difference.
I know this sounds obvious but have you changed sparkplugs?
I have a 440 Drifter that was doing the same thing. I spend hours trying to figure it out.
Had good spark on both plugs cold, sled ran really good till it warmed up, then would hardly move itself. spark tester showed spark on both sides, Finnally I changed the plugs and problem went away.
If someone had told me a plug would would work cold and not work hot I would have had a hard time believing that. But now I check that on all my sleds first.
Good Luck
Lloyd