The 426 street Hemi motors can use more compresion on todays pump gas with no problems than any other motor out there with iron heads if I was you and money wasn't a real big problem I would sell the other parts and buy a 4.25 stroke crank with BB Chevy journals, a set of BB Chevy type 7.100 long H beam steel rods and have a set of pistons made that would have a true 10.5 to1 compression ratio. I built a local customer and freind a motor just like that with a custom ground hydraulic roller cam and he loves it, it has iron heads and a dual plane single four barrel intake off of a crate motor with a Holley 850 double pumper, it is in 1968 Charger with a Dana 60 with 3.73 gear and short(28 inch tall) and a 727, full interior not cut up. It is a hoot to drive, responsive as all get out IHTHs
EDITED. The BB chevy rods and the 4.25 stroke crank won't require any grinding on the cylinder walls to clear the rods and crankshaft If you where using the same stroke crank with Mopar rod journals and mopar stock type rods you will have to grind some on the cylinder walls and also on the oil pump pickup boss to prevent them hitting
I've built a bunch of Mopar 426 Hemi and B and RB wedge motors with the BB Chevy journals and 4.25 stroke, they work great in these motors