I've been there a couple times, mounting boat anchor radio equipment
into racks. I've found a couple things that do work:
Plan your installation and mount the equipment from the bottom up. I
use spacers out of whatever material is handy, and use it to balance the
next piece of equipment.
If you have to put something above something else with a space, put two
screws in the holes just below where you are mounting said equipment.
Then use some sort of spacer (blocks, boxes, books, etc) to sit the
equipment on while you insert the proper screws.
Rotten job, but we all have to do it . . . .
Gary Hildebrand