I would guess 2600 fps or less with a 26" tube, depending on throat length it could be even less though. My Dad was getting 2350-2400 with his 338/300 WSM
in a 27.5" 3 groove Pac-Nor, but it was also throated way long for the 300gr SMK.
My brother shoots a 338/378 WBY, basically the same thing with a belt. He uses 118gr H870 with the 300gr SMK at 3000 fps in his 32" bbl. His is throated long for the 300gr SMK though, a 4.1" OAL to be exact. You might get 2900-2950 using this combo with your shorter bbl and possibly a shorter throat.