Fintan, can you share what you find out please, particularly which bell housing do you need to mate the moss gear box to the Ford block.
I'm aware that the original Ford bell housing and gearbox were cast iron, so not preferable for a sports car. I also know that the Ford gear box came with two sets of ratios, the Zephyr Six ratios would work for an Ace possibly, but the Zephyr Four would be too low/wide. Zephyr boxes are also available with Overdrive.
Zephyr Six 3.16, 2.21, 1.41, 1:1, R3.3
Zephyr Four 4.41, 2.35, 1.50, 1:1, R 4.67
For reference the Bristol BWCR 12 I believe is; 2.92, 1.82, 1.29, 1:1