The 2780's will fit under the R3T bearclaw. You will need to turn the rear filter just like on the other models. The R3T uses two bayonette type mounts at the side and rear of the bearclaw. In order to fit the rear mount you will have to push the bearclaw firmly against that rear filter. It will fit but there is significant contact with the rear filter.
I fashioned a small bracket with adjacent holes the same size as the rear bayonette hole. I then removed the rubber grommet from the hole, attached the bracket with a nut and bolt, installed the grommet on the other hole in the bracket and installed the bearclaw. This has the effect of spacing the rear out a bit, similar to using washers on the Standard/Classic models. It doesn't look great as there's now a gap between the bearclaw and tank, but it gives a lot more clearance with only minor contact with the rear filter.