My doomed 408 build

At this point in the game, the path of least resistance is to replace the hyd roller lifters with solid rollers, and run it at about .004” lash.
I’d also make sure you’re running adequate spring pressure.

You’ll need:
-Solid roller lifters
-new longer pushrods
-springs suitable to use with the solid rollers
-gaskets

Option B-
Solid flat tappet cam, edm lifters, suitable springs, pushrods.
This will require breaking in the cam with inner springs removed, and a high quality break in oil.

The springs that come with the rpm heads aren’t suitable for anything but the mildest of solid flat tappet cams.

There is an ever growing list of threads, videos, etc, of people who have had less than stellar results with retrofit hyd cam conversions....... enough so they really aren’t something I can confidently recommend any longer........ unless you’re willing to get the lifter bores bushed.

If you’re going to use/using a block that originally came with hyd roller lifters....... you have a better chance of success imo.