I believe this was discussed before either here or on P-T but I can't find the thread now.
Besides proper suspension geometry and a good surface, a big portion of that number lies in the tires.
1.2G isn't all that impressive if slicks are involved, but it still is somewhat impressive given the platform the number was achieved on.
Member KAA pulled 1.17 G with his '70 Elky, which wasn't on Air-Ride, and also on Sumitomo HTR
street tires, and had a honkin' big block in the nose. That's with a bolt-on front suspension setup and stock geometry C4L in the rear, most of the parts pieced together quite inexpensively from various circle-track vendors.
http://www.chevelles.com/forums/showthread.php?t=146629