The dozer you have, by the way, appears to be a Komatsu D-41.
The question of weight or power that's limiting you, it is more the weight issue. A 6 yd unit is a pretty good sized piece of iron for a 55hp tractor, but that hp might work it if it had more weight and traction to it. You would have a case right now of "the tail wagging the dog" if you tried to do to much with the set-up you were describing as the scraper would dictate to the tractor and not the other way around.
However, the D-41 would be perfect for this 6 yard scraper if you've got the hydraulic set-up for it.
Most tank building/cleaning around here is done with a dozer anyway, so maybe those evenings after work would be better spent plugging away with the Komatsu rather than trying a whole new rig which is unlikely to work.