There's nothing wrong (or particularly scary) about using 755 motors in a custom gearbox without a Versaplanetary. Consider using the West Coast Products
32DP gears for the first stage reduction. You can buy a pinion which presses onto the 775 shaft. With the 80T gear, you can get a 6.6:1 reduction in a pretty small and integrated package. Combine that with a pretty modest second stage reduction with standard 20DP gears, and you can get that 21:1 reduction you mentioned in your first post.
This general concept is pretty much what 254 did this year for our hanger winch.