...and no, don't use a GSR crank in the LS, you'll just lose the displacment that justified the block in the first place.
I don't see how there's really anyway that you're not going to end up having to spend some extra money. If a slight overbore will resolve the pitting (slight) the pistons will have to reflect this change, if you go the LS/Vtec route, new rods are in order.