273 rockers on 340 with hydraulic cam

ED Hamburger used to sell a shim package for decreasing the factory lifter preload when using a hydraulic cam and NON ADJUSTABLE rocker arms.
The shims were used to increase RPM range, and horsepower without switching to an adjustable recker setup. A cheap way to avoid lifter pump up. 1980 tech
The kit came with 10 each: 0.015", 0.040", and 0.060" shims
Don't know why they would be used with 273 adjustable rockers-backyard engineering?
I should still have a magazine write-up on this somewhere.