What Steve said.
It's hard to successfully take a design that was originally intended to be cast in iron and duplicate it in aluminum without major changes; there are substantial material property differences that must be taken into account. Some of these issues can likely not be designed around satisfactorily when you're limited by the dimensions and packaging and internal components of the original iron design. With such a substantial material change, it's best to start with a clean slate and design around the new material.
All the aluminum Gen-1 blocks (SBC and BBC) I've dealt with "move around' quite a bit. This comes with the territory. Valve adjustments, head bolt torques, etc, they all seem to move around and have to be checked far more frequently than an iron block.
Now if the block was originally designed in aluminum, then you're OK; the LSX series would be the logical choice. Since it was designed out of aluminum originally you don't have to worry about possible shortcomings of a "modernized" old design; since the designers are not limited by the constraints of being backwards compatible with the old stuff they can take full advantage of aluminum and also work with and around its inherent drawbacks over iron (greater CTE, lower yield strength, finite fatigue life, etc.)
Troy
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1969 Chevelle
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: LS2, 3.0 Whipple, T56 Magnum, torque arm & watts link, Wilwood Aero6/4 brakes, Mk60 ABS, Vaporworx, floater 9" rear, etc.
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