My preference and one I haven't ween mentioned is keeping the SM420 and adding a Gear Vendors overdrive. This allows you to keep the granny 1st gear, as well as split all the remaining gears since it is a true overdrive. They are expensive new, but can be found used for around $500 in my neck of the woods.

Devin

If you can't hose it out it ain't a truck

55 3100
63 Corvair Monza
64 El Camino
72 240Z
01 Suburban