missing @ 3500 or higher when @ temp

[USMC302]

You need to get a digital A/F ratio meter so we can see what it's actually reading, and change the plugs as well, just in case there are a couple fouled. Since you don't have an adjustable valvetrain, or bent pushrods, I'm sticking to my original diagnosis of a collapsed lifter. Lifters are fairly cheap but, not always fun to replace. Sounds like you have two different issues unfortunately.