the multi layer will actucally lower compression a small ammount because you are increasing volume in the combustion chamber during the power stroke. thicker h/g means more room between head and block, compression ratio is how much you are compressing the volume of air/fuel that was in there a b.d.c. (bottom dead center, specifically bdc of the intake stroke in this case.)

as for a crank get the one the rods you have fit on. i would stick with ls. more stroke, and like nurve said get it balanced and blue printed... you can rev that balanced ls crank as high as if not higher than any gsr or b16 all he while getting more stroke.