Alright, first, you don't need a "+P" load. Any 240 grain hollow point or soft tip, doing about 1300 fps or faster (as listed on the box) will work fine. Remember the speeds listed for the ammo since it's a famous handgun round, are for either a 4" or 6" revolver barrel (whichever the manufacturer used as a test-bed). You with a rifle and no cylinder gap, and that longer barrel, will capture and use all of the gas pressure that's possible to capture so will get plenty of velocity. Find the brand and load that shoots the best.
As for a "neck shot" if you want DRT, then I'd suggest a shoulder shot instead. The problem with the classic neck shot, while when done without problem they work very well, it's easier for the animal to move the neck and thus you get a real tracking problem on your hands, sometimes. The shoulder shot pretty much pole-axes them, especially with that large slug.
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My .530, 225 grain soft lead round ball comes from my rifle at just over 1500 fps... and although it's larger than your slug will be, it's lighter and has a crappy ballistic coefficient so it drops below supersonic at around 80 yards...so if mine when it hits where indicated above literally drops the deer in their tracks at 100 yards, your slug will definitely do the same, as it will likely still be super sonic on impact, especially if you get that deer in at 75 yards or less. Don't plan on any second shots, just as I mentioned, get a very accurate load and put the bullet in right spot.
LD