Paul
You hit the nail on the head with the diff ratio thing.
The plan to use it was established back when I had intended to run this engine as a stroker with the A14 crank, but to be honest, the 56A gearboxes don't like the higher torque engines too much, so I have reverted to plan 'B' with the short stroke crank & I have given some thought to the best diff ratio.
Still, the 3.7 is not a big step up in ratio from 3.9 & the A13 will still pump out a bit more torque than my old 1200 GX did in the B10 sedan. I ran a 3.9 diff in that [from a 1200] & towed a small boat, fully laden, well up the north coast & back without problems, even outran some wannabe in a near new VC Commodore up O'Sulivans Gap Pass with the boat in tow, so I might be getting worried about nothing.
I also remember that it handled second gear takeoffs without any problem at all, so that's got to be a good sign. In fact, come to think of it, first always was a little on the low side, but good for traffic.

My driving style is kind to clutches & drivelines, so an easy takeoff with a healthy rev-out in the gears should keep the Adrenalin flowing & still allow me some rpm reduction at highway cruise.

EDIT
First gear ratio's
F4W56A ......... 3.760 to 1 [2nd gear 2.170 to 1]
F5W56A ......... 3.757 to 1 [2nd gear 2.374 to 1]
F5C56A .......... 2.676 to 1 [2nd gear 1.691 to 1]
As you can see the Option 1 box has a taller first gear than the other two, but not as tall as second gear in either of them. I guess I will need to suck it & see, but trust me, I am really looking forward to seeing just what this gearbox can do.

As for a cam, no decision yet, but something that puts peak torque up around 3,000 to 3,500 rpm might be nice, however your thoughts, as always, are welcome & appreciated.