Stupid question but I have to ask. Are you sure your alignment is off in the right direction? You say it is 75% open with the GB up against the barrel shoulder. A brain fart could easily have your problem bass-ackwards and it needs to be backed off the barrel shoulder to align it properly. I have seen people slam the GB up against the shoulder several times and cause a partial blockage of the gas port, but it is really rare, especially with decent makers, where the gas block or gas port was mis-drilled or the barrel shoulder is in the wrong place.
If you are correct though, and another gas block does not solve the issue and you don't want to send the barrel back for replacement, yes, you can use a belt sander and take off some off the face to get it to align but I would quadruple check to make sure the grinding off the face of the gas block will solve the issue without creating another, such as setting back the gas tube too far and causing it to bottom out in the gas key, which might happen if the gas port or shoulder is indeed in the wrong location. My money is on a brain fart that has you thinking backwards and moving it off the shoulder will correct it. I've been wrong before, in fact, several times just today, so good luck.