I've always heard that it is the 16:1(fast ratio) manual box. In fact, it is the box that came on my 66 GT. I have since switched to an HCC-AW box ( which is also 16:1) and the power steering set up from the 66 coupe I sacrificed to save my fastback.
Glenn Morgan: 66 GT V-Burgundy Fastback 351w+toploader+9 in. TracLoc. Started out as a rusted-out Chicago-area crusher. After sacrificing a solid 66 coupe for its sheetmetal sub-assemblies, I have one solid (and expensive) work in progress!