Counter rotating? *scratching head*,,,why would a manufactor
build something like that. Not trying to be impertinent, just wondering. Special purpose machine?
If the blades have a bend in them to create a depression in the center, which I think most of them have, you won't be able to reverse blades. You'll have to grind a cutting edge on the back side of the blade. Can be done, for sure, but take some quality time with a grinder.
I'm not sure about the effect on the gearbox. Probably should be ok(notice the car mechanic cya modifiers, probably & should*G*). I think if the input shaft comes into the gear box half way up the front of the box, it'll be ok for sure. If the input shaft in down low, like a car rear end, it's got helical gears and running in reverse may not be a good idea. But it's probalby got straight spur bevel gears, since they're a lot cheaper to produce, and they don't care which way they're turning.
Dave Perry