Small Block Full Length Header Options - Ground Clearance
Doug's!!!
Allegedly the Doug's have about 3/8" more ground clearance than the TTI's. But I haven't personally measured a set of TTI's to find out if that number is accurate.
I can say though that looking at the Doug's on my car I don't see how you could get any more ground clearance with a set of full-length headers than what they give.