If you plan to rise the original gearboxes (i.e. you didn't throw in the low profile ones), I might suggest to just rise the front of the gearbox and keep the rear at the original height. That will spare you a lot of rework on the gearboxes and on the upper hull to make room enought to close the two halves. You will just have to cut a bit from the top of the gearbox side sheets, but with a dremel cutting disc it is quick.

Then you drill two holes each side for the gearbox cover pins, and rotate some degrees the covers around the fixing screw in order to re-center the shaft in them.

One friend of mine followed this way and he obtained the same result with less trouble.