They both have the same general design of mounts, but the chances of it being a direct fit are low.
The BT820 was only used on the B26, they do not list a separate mount, as the mounts are part of the ROPS and framework of the B26.
The BH77 is actually smaller than the BT820 so you would be getting a larger backhoe that might over power the tractor.
Hopefully your modification tools and skills are up to par, and then yes I think with the right mods to the mounts it should work fine as far as fit.
One thing to keep in mind is that your tractor has less Hydralic flow that the B26, looks to be only 2 GPM differance but that little of a differance can make a big BH a lot slower.
