I would build it with the DOHC, diff and gearbox that you have, then swap out after IVA.
The diff is not as much of a problem as the gearbox, diffs are easy to change and the 3.9 gives very good acceleration, the main problem being the very low 1st gear and the big gap to second in the Ford MT75 and Type 9.
The rev limit of the engine is another factor, down load one of the many gear ratio programs and have a play with wheel, tyre sizes, diff ratios and gear ratios.
When you get the lower gears working as you want the higher gears are far to long and vis vers.
Ideal ratios are 1st 2.8 to 2.5 2nd 1.8 to 1.6 3rd and 4th are ok 5th could do with being about 0.9 not the 0.825 that it is.
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