I have a hydraulic lifter slant 6 (1983) and was wondering if I have to get shorter push rods if I milled the head 0.100"? The rockers are not adjustable.

I would say yes.
Shorter pushrods or shim-up the rocker arm assembly.
The best deal would be to have adjustable push rods but I don't know of any that allow the oil flow thru the pushrod while being long enough.

The best way to "set" the hydrualic lifter is to give it just a little "preload", enough to take-up the clearance and keep things quite but not a lot, so they don't 'pump-up' and hang the valves open at high RPMs.
DD