Should I make the swap to G3?

You failed to say what year and model car you have and if it has any value numbers matching etc. You also did not say what your expectations are. What will you gain, less weight? More power? Personally if I was swapping to a G3, it would be one of the larger displacement engines, not the 5.7.
As for the k frame, that does seem rip but if these new swaps are the new trend, I would expect it to add value to that year k frame. I would make sure that k frame was pristine for that price, original paint, no rust pitting and no dents in the bottom.