I never heard of any swap on GS300 except for Supra TT 2JZGTE engine, unless those are for GS400/430 only. I think V8 vs. I6 swap will be much more complicated as you need to change all the ECUs and harnesses, basically to re-wire the whole car, adjust all the circuits. I don't think it will be any cheaper or easier than turbo/supercharger tuning.