What do you have in the spk's currently, what type of footbed. Your foot could well be pronating and expanding in the boot, which makes it feel to small. Do you have any good boot fitters that work in your shop. If you do have once of them see if you are in a neutral stance in side the boot.
If you do need a bigger size, then befire ordering anything, try some things on with the boot fitter. Obviously if the SPK fits well but a bit tight, its unlickly a FT will fit, as the classic shell is much tighter, and the influence shell is still a smaller volume then the SPK. Atomic may be an option though, as may the impact or mission boots from salomon. Head vector may be worth a look too, infact there are any number of boots that go that big, its just a case of knowing which will work for your foot.
But as i said have a boot fitter see if you can get the SPK fitting better first.