I think that in SSGT you do not have to run a non original combo. He could still be running the 383. Some guys change from SS to SSGT to avoid heads up runs but as a #2 qual I don't think Mike needs to worry about that. Some engines carry a different HP factor in GT than regular SS so although a 10 lb index in GT is quicker than a 10 lb traditional SS index with different HP ratings you can sometimes run lighter in SSGT than SS.
All that said, still don't know what engine he is running.