[Bored&Stroked]

Given that the valves are angled and the actual location of the spark plug hole and that a standard head gasket is about .052 thick, are you measuring the clearance at the location where the spark plug is oriented to the valve (with a head gasket in place) - or the top side of the valve (toward the intake) - which has a different height?

Consider that your particular block, cam, head, valve seat depth, valve type combination is specific to your particular motor - nobody can tell you exactly what to do. The answer is "it depends" . . . on a lot of different factors. In the end, the dimensions are what drives your options - plain and simple.

All you need to do is clay the top of the valve at full lift - with a head gasket in place and a couple studs/bolts and see what the exact amount of clearance you have above the valve in the exact location. Then, you know what you have to work with - and can determine how deep the plug can go.

Once you know that, then you can determine which plugs you can use (given the length, heat range and electrode type) and how many potential spacers (or not) you need to utilize. You'll have options as to how you want to address your specific dimensions. If you need a shorter reach plug (and then have threads showing in the chamber side), you can chamfer and also maybe add a power-slot in the head as Gary has shown.

There are lots of ways to address any potential issue - just decide which plugs you want to run and therefore what you need to do.

Good luck!
B&S