I ran a 5ft BB on the MFWD (4x4) 750 I used to have and it pulled it pretty well. Of course, the MFWD really helped a lot, along with the ag tires and wheel weights it had. As with most tractors, you can eventually find the "stall point" if you load up the box with enough wet dirt/gravel/etc no matter what size the BB is. The trick is to try to get no more material in the box than you know will probably bog it down to keep from spinning the tires.
If the OP's 750 is 2 wheel drive, has turf tires, or no wheel weights, his stall point will definitely be a bit lower. He could still run a 5 ft BB to be able to cover the tire tracks nicely, but just have to watch it to not "overload" the box, or dig with all 5 rippers in the ground. Operator experience will go a long ways to using any tractor and boxblade to its fullest capabilities.
I did run a 4ft bush hog on the 750 though, that was really all it wanted to handle too. The 750's I had did not have a loader on them so the front end was fairly light with the 4ft bush hog on the back. I just think a 5ft bush hog would be too much for that 21hp tractor. I know the OP didn't mention getting a bushhog, but I thought I would point that out just in case he did get one at some point.
The 750's are actually great little tractors though! I've actually owned two of them, the first one was '81 two wheel drive 750 that I had for about 5 years, then had a chance to trade up to a '84 4x4 750. Liked them both, but eventually decided I wanted a loader so I sold the '84 and got the 790 I have now. I hope you enjoy your 750.
