I don't know that you are goping to be abvle to find a generation1 tube in a size that will allow it to mount behind the M2 on a flattop upper or a rail that's stable enough to mount the two w/ the Aimpoint out in front of the receiver.
The newer tubes and scopes AN-PVS14, 18 etc. are samll enough to fit behind the M2, and can also be used as a head/helmet mounted monoc. but, you're not going to be able to use the head/helmet mount and the M2, in conjunction, w/ any proficiency, because of head alignment and positioning issues. You would need a head/helmet mt. settup in conjunction w/ an IR target designator (IR laser) like the PEQ2/ATTILA/CORSAK etc.
Others here, WES at MSTN, [url]www.mstn.biz/[/url]doesn't sell NVDs any more but has a lot of history w/ them and a lot of understanding of their capabilities, alimits and liabilities.
Check w/ him, he's not selling them, and does a lot of business here, and elsewhere, so he may not be able to make the "Q" a priority but he's always gotten back to me. Others here, are also very knowledgable on NVDs, try KevinB, too now that I think of it. Sorry in advance guys, hope you don't mind, I'm trying to help him out.
Hope it helps.