273/318/340 all shared the same stroke and bearing sizes. 340's had heavier counterweights to work with heavier rods. They can be mixed and matched, but you should probably have a mixed cubic inch assembly balanced to make sure there are not vibrations. Magnums also used different rods (smaller at the wrist pin, lighter pistons) than LA's, to that's another reason to balance if you are mixing Magnum and LA.