596 heads spring installed height too low

First, the springs don't affect the installed height. Vice-versa, your springs require the 1.750" installed height so they don't bottom out ("closed coil") when the cam is at max lift.

I assume you measured the installed height with the rocker assembly out. 0.100" is a tremendous difference. Only a few things affect that - the length of the valves, how the valve seats in the head (do you have hardened seats?), and any shims under the springs. It is a bit tricky to measure the spring length installed.

It has been a while since I installed and checked my springs. I used a vernier caliper, with the plunge end sitting on the top of the shim under the spring. I adjusted shims to get the height correct. I forgot the shim increments (maybe 0.005" thick?).

I expect whoever assembled your heads would have checked the spring heights. That is a normal part of the job, since they must shim the springs. One guess is that you are checking with the rockers and pushrods installed and the pushrods are holding the valves open. Maybe they are too long. You could tell by looking at the hydraulic lifters. Are they pushed way down?