Shims under valves springs

[Olivieeeer]

   Hi ! I'm preparing a new head with enlarged and polished ports, enlarged manifolds for 36mm carbs, and 1mm milling  ( ~ 40 thousandths ). I think the engine will rev much better, and was wondering if it will be pertinent to add some shims under the valves springs for compressing the springs a little bit. I read this method in a modifications book. Then I took somes measurements :

- ( the big ) Spring lengh : 42mm
- Valve " tail " lengh : 40mm
- Stock shims thickness : 2mm

The spring mounted is 36mm long.

Compressing the spring to the max, It is 23mm.

When the valve lift is max ( 8.5mm ), the spring is theorically 36 - 8.5 = 27.5mm long.

I have some 2mm shims ... if I'm not wrong, I have enough space for them. Note that there's not any problem with the little valve spring.

Some opinions ?