Here's the math:
4.342 bore 3.38 stroke turns into 400.4
Each cylinder = 50.05 = 820.5cc
Divide by r-1 to get volume above the piston for compression ratio r.
820.5 / 7.5 = 109.4cc
Assume 88cc head, 5cc gasket steel shim
16.4cc needed.
(16.4 / 820.5) x 3.38 = 0.068 Piston top that far down in the hole.
9.98 - 6.358 - (3.38 / 2) - 0.068 = 1.864
That's the compression height to give you 8.5:1
Both F-M and Silvolite have pistons with comp height or 1.812 or 1.813 giving you 7.72:1.
The KB 240 piston has a 1.908 comp[ height with a 8cc valve notches, gives their calc 8.9 CR with 88cc head. Campbellenterprises has them on sale for about $450 a set.
By the time you do all the milling and buy your pistons, minimum $225 for NOS on the 'bay, you have about paid for the KBs.
Otherwise you're SOL.
R.