You need to know what lifters are installed Standard, anti pump ups, roads etc? some are adjusted with a preload some aren't. Chrysler used different length push rods and valve lash caps not adjustable push rods at the time. I only use adjustable push rods to check rod length I never run them on any engine I build! But thats my choice do what your comfortable with
Also lifters on Chryslers were clearanced and measured at full collapse like a ford F.E. side oiler. I would have thought your engine builder would have used (or talked you into ) adjustable rockers and the right length custom push rods. I also use the valve lash caps with roller rockers or standard rockers on Chryslers especially with stainless valves ( because of the softer stainless compared to steel with hardened tiped valves) and it gives a little more surface area for your roller to ride on. Its just the way I do things
Adjustable push rods may add to your problem some of the cheep brands flex at high rpm and pressures! Try setting your rockers at .000 lash and see if the problem goes away or appears at a higher rpm if it does you need better lifters or redo your valve geometry (and I would suggest a builder with more experience with older Chrysler engines) If you cant adjust them you have to change your set up so you can!!!
These are just suggestions: Jester
Take some pics of your valve train from different angles so we can see the geometry and set up it would help