Guidance on Adjusting Main Shims Insutu

Evening guys,

Fired up my 29 six cyl today after head replacement (Thanks Karl!), new timing chain, new rings, wrist pins & bush, and rod ends reshimmed almost snug.

After a first couple of short runs, 10 minutes total, , had a pronounced thud, thud, thud. Appears with more compression I now have a sloppy main or two. Engine is still on the stand so pulling the cap will be done from beneath. Tranny is attached to help with rear sag.  I made a tool last year to help pull the main caps on an engine with catastrophic freeze damage. So I've done that plus have spare shims and good caps. Other than that I have not adjusted shims on the mains yet. I'm hoping the center main is the thumper. Any advice welcome, please.