...1.6" ?? I think You might have Your Numbers Confused. Did You mean 0.016" ? Or 0.160" ?
It would be Cheaper and Faster to Just Purchase the Correct Set of Springs. AND You could go with a 'Bee-Hive' Design, the Latest Technology.
You could also go with Longer Valves (Stems), But that would Drastically Change the 'Rocker Arm' Geometry! Perhaps THAT is Exactly what IS Needed! In the Photo, It looks as if You are already Set-Up to Check the Geometry.
Machinists will Add 'Spring-Seat' SHIMS, to INCREASE Spring Pressure, and to Protect Aluminum Seats, from Steel Springs. Milling the Seats, to Decrease Pressure, is Not Necessary and is NEVER Done, in My Opinion.