the leaf link you mention is called an anti tramp bar setup in the uk. as you say the front half of the leaf spring effectively becomes the lower 'link' of a 4 link. they even came as factory equipment on some mid to late 60's fords here.
neil.
the 72 Motown Missile 'Cuda was originally built with one and tested extensively. We built a similar version in the early-mid 70's. After several test sessions, we removed them and went back to the tried and true Mopar Super Stock springs......... and never looked back.