If you go big like the 797 you can use eaton hydrostatic trans like:
http://hydraulics.eaton.com/products/pdfs/07-403.pdf
or maybee go for a rear axle ready like:
http://hydraulics.eaton.com/products...LD-MC002-E.pdf
I was concider this trans for my D8R but it was to big to be built into the model. Best part with the eaton trans is that you have speed control, reverse and braking built in. It "only " to connect the engine to input shaft. Then of course some small metal fabrication.
Check video on other tread about 988, there´s the A35D dumper project in the end. This dumper is built with mecanical driveline (incl gearbox, 2 speed and reverse). Power to cylinders can easily be done with a small gearpump, then you don´t need big oil tank for the transmission
Keep up modeling!
Dan