IIRC JohnRR priorly posted that a 68-69 piston has a 2.030"CH & is ~.050 in the hole (as we know block deck heights vary) but you might plug it all into united machines' calculator. You know the std bore and stroke & the gasket dimention is available along with a good guesstimate on the 906's CC's & I'm assuming the piston has reliefs (find that info too) then let your fingers do the walking. also see if there are differences in 6pak pistons (not my area).