Those variable double pumps look like a bargin at $129.99. I have spent a few days figuring out how I could use one on my project but can't get around that I would have to build a separate hydraulic system to operate a dump bed and blade. It seems ideal for your machine. A fellow on the machine builders network ended up having to encase his chain drives to keep rocks and debris out. I was thinking that with a looser fitting chain and a spring loaded idiler would help your chain drive to handle some debris? From what I have learned, with your 12hp engine, it should really load up with both pumps putting out 6gpm per side. Since the sound of your dozer engine is not changing in the video, you could likely go with 1:1 gearing. Of course then you would need a suspension seat.