I just found out I have the same problem today. With a 7.5" barrel, and a 3.25" blast can, it makes the assembly too short for the 10" ALG EMR V2.
The 7.5" barrel only sticks out of the upper 7", and the 3.25" can threads onto the barrel 1/2". So the two together only equals about 9.75"... F me... I did not know all of this until I had the parts in hand and was measuring to see where the 10" handguard would land.
My plan has failed. I don't want the 8" keymod ALG makes, but wouldn't mind Mlock of the EMR V2 because the top section has a built in rail for irons.
I just might order the V3 and hack the last 1-2" off. Then mill the top rail off to make it a lower profile handguard so I can wrap my thumb over the top while using the Magpul ALG grippy for Mlock (whatever they call it). I'll call ALG and will ask if they'll ever do a 8" V2.