You may be correct on the valve stem diameter , I've never work on one of those 354 Hemi truck motors, only heard from H.D. mechanics that did about those details. I've owned and ran a bunch of different early Chrysler passenger car Hemi motors, even seen some of the industrial 354 hemi apart in a shop years ago but I didn't measure or work on the parts myself I have seen a bunch of the plain no name(truck and industrial is what I was told they where off of) stock steel valve covers for adjustable rocker arms at a bunch of different swap meest in SO CA years ago. Those valve covers where normally only used on the solid lifter motors Most of the old H.D. gasoline truck motors came with sodium filled exhaust valves, I even heard some of the later H.D. truck motors came with inconel exhuast valves, maybe not so