Looks like it has pretty much been answered already but I'd shoot for 65 MPH cruise right at 2K RPM. 4.10 should put you very close.
Of course you want to make sure your T56 1st gear is friendly to that 4.10, what is your T56's first gear ratio?
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1969 Camaro/ Tom Nelson TT 434 / Wayne Due C5 / DSE QLink / and a bunch of other stuff...