the shim would go under the rubber block at the rear portion where it meets the ski. basically a piece of old hyfax, old belt, or anything hard can be used. it needs to be permanently attached by glue or screw.
remove a ski and the rubber block. see if there is a letter on the block like an "S". with the "S" rubbers, you will need a thinner shim like 1/8-3/16. the non "S" ones will need a bit thicker.
before you start, get the sled on flat, level ground. look at how and where the carbide is sitting in relation to the floor. the front portion of the carbide should be about 1/4" off the floor. this is what your shooting for by shimming the spindle. there is no cut and dried "this works for that sled" thickness, its trial and error.
cut yourself a few shims about 2" long and 1" wide in thicknesses of 1/16" increments: 1/8, 3/16, and 1/4. now using the trial and error method, install a shim and measure the distance from the floor to the carbide. your shooting for about 1/4" off the floor. too much shim= too high, not enough= too low.
find what seems right and glue it on. rtv silicone works good or any other type rubber adhesive. then you need to test and see if if it works. you may have to play a bit to find your happy spot.
also, Bergstrom used to sell these shims so you might opt for that route.
as for suspension tuning, increasing preload on the ski socks, reducing preload on the center shock, increasing preload on the rear shock and limiter strap adjustments will all affect ski pressure. make one change at a time and test. and, as always, note your starting points prior to adjustment for a return to original positioning if the adjustments dont suit your needs.