I'm not sure what modification is required to fit a GTR bar to a GTSt. Mine has had 3 on it (the one that came from Japan with it, and 2x replacements repairs due to arseholes crashing into it!). They just seem to go on without a struggle. Of course, my car also has the GTR headlights, indicators, grille and bonnet, just in case any of those affect the fit, particularly towards the outer ends.
You can buy GTR bumpers new. Not cheap, but readily available.
As to the JSAI etc one. No experience, but they look fine.