Beat ya to it by a couple of months
www.ar15.com/forums/topic.html?id=105159
Seriously, see how useful the search feature of this site can be?

Quoted: Hey Sixgun, You can really make it your bolt action M1A, just add a 20 round trench mag (made for Mauser rifles). Just pop the plate off the bottom, they pop right on. I think Sarco's got'em, but their pricey.

I have one of those, and it will attach BUT: the follower and spring (hell, the whole mag) are designed for 8mm Mauser ammo.
To make the mag work for .308, you would need a shorter follower, a narrower spring, and you would have to weld/braze/etc. a spacer to the front wall of the mag so that the .308 rds would not slip forward and hang up during recoil, etc..
In addition, you will have to remove the brazed-on spacer installed in the original 5-rd mag well which was originally installed for just that purpose.
There's lots of cheap .308 20-rd mags out there.
I'd pick one with a follower as close as possible to the original 5-rd follower, and adapt the follower and spring to the Mauser mag.
Personally, as the mag is not readily detachable, I think you would not be creating a new, illegal hi-cap mag.  But, since I'm not a lawyer, I'd check on the legalities first.

Oh, and Sixgun, PLEASE thoroughly soak the front sight post and locking screw with serious penetrating oil before trying to move them.  It's been reported that some FR-8 sights, which have very fine threads were rusted tight.

Quoted: Quoted: see how useful the search feature of this site can be? It is great on the rare occasion that it actually works.

I dunno.  Seems to work for me.  I used it above to find my post I linked.  Sometimes you have to fiddle with the settings.  It's not exactly user-friendly, I'll give you that.  But if you learn its' peculiarities, it will work.  At least that's my experience.  YMMV.

Six, I haven't tried the brake, so I can't say one way or another.
As for removing the FS, it unscrews normally.  Use a strap wrench or other appropriate tool to avoid marring the finish.

Those half-round cutouts on the front of the FS are a wire-cutter.  Affix the wire in the two cut-outs, push tight, and fire.  Neat. huh?

You might consider a slip-on recoil pad from Pachmayr, or a wearable pad such as offeered by PAST.  Natchez Shooters Supply and Midway offer them, IIRC.