I had the 20:1 box (actually the 20:1 sector shaft back when MP sold them) on a 70 Cuda. Manual Steering, 14 or 15" wheel (the basic e-body two-spoke). It was a bear at anything less than a crawl, but GREAT from about 3mph up.

I agree that a 16:1 with a 13 inch wheel will be STIFF.