JB, the spec I contributed was meant to put your mind at ease in the event you had the input with the long nipple on the end. That long nipple should still have enough space as it goes through the pilot bushing and not bottom out in the Scat crank.
Regarding input shaft length on the big I/O toploaders I get 6- 6.125.” This is from trans face to end of nipple. That would work with 6.4 spacing. You just need to be certain that the splined section of the input is not right against the bushing. Sometimes tolerances stack up against you. A “proud” pilot bushing could cost you some wiggle room. Measure very carefully.