As a prior service Marine, I am considering joining the National Guard. Since I am 40% disabled, would that interfere with my drill pay?

It will interfere with drill payments. According to 10 U.S. Code § 12316 and 38 U.S. Code § 5304 you have to suspend either your disability payment or your drill payment during the time you are active since you can't be paid for both at the same time. In short, the law states you can collect a pension, retainer pay, disability compensation or other earned pay, however, you must choose to suspend that benefit if you are called up for training or to active duty if you want to receive active duty or training pay.

Because you can’t be paid by both the military and the VA on the same day, you must choose which pay you wish to receive, and which you wish to waive. You can do this at the end of the year with VA Form 21-8951, Notice of Waiver of VA Compensation or Pension to Receive Military Pay and Allowances. You must fill out this form each year in which you receive VA service-connected disability compensation or pension benefits and you serve on paid status in the Guard or Reserves.

The VA should send you this form during the first quarter of the current year, for the previous fiscal year. If you haven’t received a copy, you should request an annual waiver from the VA. However, keep in mind, this is up to you to complete and return. If you do not receive a form from the VA, then you should initiate completing the form on your own.

Sgt Tylor Dewett also, before you dive head-first, make sure you contact a good recruiter to see if you will be able to get a waiver to get in, too. It has taken me 17 years to find a recruiter willing to fight to get me back in w/ my tiny 10% rating.