valve spring retainers

i will see if the speed shop will take back my new 911 springs aback (never used) and get some 926 springs then shim the valve springs to as close as i can get to 1.80 . my installed height now with the 911 springs range from 1.830 - 1.870 so the 926 springs (installed height of 1.80) should put me where i want to be. the engine is a mild 1968 383 with a new 68 RR 335hpMP cam thanks