because, it always seemed that those engines really needed some taller gears, that the regular 3-on-the-tree transmissions didn't provide. plus, there was a lot of space between those 3 gears. it'd be interesting to compare 1/4 mile times between two identical fords with both transmissions installed.
i know there were some overdrives available, back then, but a 5-speed w/od would have provided a lot better get-away, plus low-rpm highway cruising.
i'd love to have my old '51 flathead 6-powered mainliner today...i'd definately put an S-10 5-speed in it!
it seems to me that your rear gears are awfully tall for an overdrive tranny. i would have thought a set of 3.73:1, or even 4.00:1 gears would get your engine closer to the optimum rpm range.
