Mike,
I am sure that you are aware of some Ford 9" facts but it is worthwhile for you to review before you get too far along.....The 9" Ford rear end has a considerable offset (vertical) of the pinion centerline so if you have already planned on a pinion centerline (vertical) using a GM rear axle, the offset difference should be evaluated.
As far as the drawing side and supplying same to a gear manufacturer, it is perhaps possible to get those type of drawings from an existing manufacturer if you might ask them. However, having addressed the problem is far from solving it. Perhaps it is better to discuss the issue in detail with a manufacturer such as Meritor that indeed might have a ring and pinion already done that is very close to the ratio you say you want. Just a thought to perhaps ease the angst a bit.
Stay safe and clear of the snakes. Both Mr. No Shoulders and the two legged kind as well.
Best of Luck in the coming New Year.
Regards,
HB2