Usually a valve adjustment is not necessary due to the hydraulic lifters and the rocker design. But I had a bad rattle and a year ago so I adjusted them to zero lash. Put it all back together with the same gaskets and developed a leak. So this year I opened it back up to put new gaskets in and found one of the four valve settings was to great so I readjusted it. Sounds much quieter now and doesn't leak. Still has that Roadstar upper end rattle though. Most of that is side slap of the rockers. You can move the rockers left and right and see that there is some slight movement.
I seen a write up on shimming them once. I'd be concerned somewhat about binding, aluminum expansion being what it is in that temp range and all. I'd have to do some serious egghead math before I did that. Mine is quieter since it passed 50K miles for some reason. Not complaining.
Thin nylon washers/shims would quiet it down don't you think? Not so much for filling the space but for eliminating metal to metal slap. Who wants to try this next time you are in there?