Lots of things to take into account, springs shock absorber travel, maybe struts to the back of the seat for fully floating, etc. I have an AVO monoshock and with the 3" longer seat you have to take into account that the rear guard/lifting handle will likely hit the back of the seat. I flipped the lifting handle over so that it is an inch or two further back. If you do this you need to weld a new fixing tab onto what has now become the right-hand stay. Best if you can remove the springs and move the rear wheel up and down by hand. You will then know what movement you have and what the limiting factor is going to be.
Also the "B" RFM is slightly shorter than the "C" type so you will have a little less movement.