Hey,
The Cage was not made to Cams specs as far as I know but i would take a guess that it would easily exceed the strength requirements etc. to do so. I've lost my CAMS booklet so CAMS Schedule J specifications means nothing to me at the moment, sorry. If you were to come and take a look at the workmanship and materials used you would appreciate what i mean by "easily exceed the strength requirements".
The Cage was made for a Gts-t but will fit both as I have A Full Safety 21 GTR cage in my Gts-t.
The harness straps are 3 inch wide for that improved safety and comfort and you can defiantly use them with stock seats as I've been using a TRS Harness with my GTR seats. Just run the harness' to the mounting points on the cage or to eyelet bolts screwed into where your rear seat belts bolt into.
Hope that helps, Cheers