I've seen quite a few stock case 18dm's. Years back billet cases were more money than today, and years back the 775 class was HUGE so the weight difference was worth the hassle. These days, probably not so much. Its very possible, and provided you don't have a major failure will last for several years, but the first time something goes fucky they don't have the strength to stay together. IF you are going to go the stock case route be sure to use a builder who does GOOD case work! Titan, Redline, WK would be my suggestions but don't be surprised if they try to sway you to billets.
Yes, an 18dm on pavement will live on a billet second. You will have to soften the clutch up for holeshot which makes things far easier on tranny parts. Lastly if your worried about people recognizing the billet cases, spend some time smoothing and rolling the hard edges that are always seen, then sand blast the fuck out of them to loose that smooth gloss. Stock stator and clutch covers on each side and big carb bowls to hide the rest. I've seen 2 done like that and both took a good look too figure out they were billets.