I'd agree with Inspyral that it'd be easier to swap an LS crank into a GsR, although maybe more expensive. If you own an Ls, try and sell it as a swap. You can pick up around $2000 for an Ls swap fairly easily, and buy a GsR swap for $3500, the difference would be much cheaper, and you'd make more power and have more reliability than some one-off hybrid freak motor crafted from all sorts of different parts.