Kyle,

What were the results of your diagnosis as described in post #12?

 

I had the same issue on my 400 several years ago and id just like to share how I fixed it. Pretty much a stock rebuild, (in 1996) but I used a Comp Cams 268. I also used the Crane screw in stud kit and Crane BBC roller tip rockers. I know they aren't the same ratio but it worked. In hindsight I wish I would have dropped the cash on a 1.73 set I probably would have got a few more ponies out of it. When I adjusted the valves I tightened until there was some resistance on the PR as I rotated it with my fingers. Id roll the engine and recheck them all making sure the PR would still rotate. Then locked it down. Ran that engine for 60-65K with no problems maybe more now but can't say because sadly, I sold that truck after having it for almost 30 years. Not trying to steer you away from the other remedies you've been getting as they sound very solid. I just wanted to share. Oh, and I never spun that 400 over 5K....and it had no problem getting there when I did. Peace.

 

You can check the cam lobes with the cam in the engine, but you need a specialized indicator gauge. You spin the engine and the gauge tracks the lift of the lobe.

 

Uhhhmmm...no, the OP doesn't need that specialized tool....all he needs to do is pull a couple of the offending lifters and check the wear on the lifter's foot. There's plenty of room in the heads to yank some lifters....if he can get 'em out.--as in a mushroomed foot........ just don't mix 'em up.