596 heads spring installed height too low

I dont know if i have hardened seats. There was 1 shim in bottom of every intake valve spring. Also dont know what is purpose of that.

I measured the installed height whit heads out of engine, no rocker things when measured. i have read that 1.650" is stock install height. and i know springs itselfs dont affect installed height. But if i put spring lets say 1.800" istall height, seat pressure is not what it should be, right?

Where to get retainers that raise installation height? Is there possible that the rocker arm hitting retainer, because retainer is longer?