Okay,first things first. Yes, it will change the timing but not in the simple "twist the distributor to fix it" way you might be expecting. Reducing the thickness of the head will have the effect of "lengthening" the timing chain (the chain won't actually be longer but the gears' centers will be closer together resulting in the same effect) which will be retarding the valve timing. The amount the crank to cam gear is changed will determine if you will need an offset key. You won't know until you've assembled the engine.

There MIGHT be a formula which could be used but you have too many variables involved (stretch of chain, wear of gears, etc.) to calculate with any guarantee of precision.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now