JP,
As stated before, your tactics will be the same, but some of your techniques will need to be modified. The are only two real differences between your M1A and a carbine and that is length and not having a pistolgrip. Length will be a factor because when you switch to a more squared-up stance, the distance you can project your support hand along the stock of the rifle will shorten. It's not that big a deal on a 16" carbine, but a 22" rifle changes things. There is the feeling that you have this long rifle out there with no support, but it's probably more of a comfort issue rather than an actual control issue. Don't worry that your support hand is not fully extended along the stock. Place it where you are comfortable. The bigger issue I find is the lack of a pistol grip. While I can still do a pretty good squared up stance with a straight stock setup, it's definitely different. I feel that my wrist is more along for the ride as opposed to the pistol grip where I can actually get in behind the pistol grip and absorb some of the recoil. It's something that takes me a couple of rounds to adjust to the different feel, but in short order I feel comfortable again.
Don't look at the squared up stance as replacing the traditional bladed offhand shooting position; it should compliment your skill set. For tactical problems that require more precision, continue to use the bladed stance. If a situation requires more speed and close quarters skills, then consider going to a squared up stance.
The more techniques you have in your skill set, the more options you'll have to solve any tactical problem that arises.
With respect to your sling issue, I would consider using a Viking Tactics sling. It's a traditional two point sling that has a quick adjustment feature. You can continue using it in the traditional sling manner or you can quick adjust the tension to give you more freedom of movement or quick adjust the slack and tighten up the sling for precision shooting. I find the quick adjust feature to be indispensable.
USSA-1