Chevy II GT as in '62-'63 Fastbacks? Think could have had anything but SB 377 was the/a hot item. 4x4 would be trying to fit a 4" stroke in there. Don't think that is a good idea.
That Hot Rod article is short but nice info. The video adds little more. Too bad they didn't run with the original manifolds/headers. That engine seems to be a stock MkII 427 HiPerf by the power output. Video said power was leaning over ~5,700... think should be like ~6,300 from data history.
Hot Rod MkII Article/Video
Paul