The Granberg Alaskan MKIV mill is what I have. (Mine is a different model designation because it is rather old, but is the same design. I bought mine 2nd hand from Scott Smith). It attaches to the bar at both ends. With it, you will need the mill length and your bar/chain to be 6" longer than the widest log you plan to cut. They come in different lengths, or you can buy longer guide bars if needed. It works well but will tax your endurance on large logs. Be prepared for a copious amount of chips (aftetr a few large logs, you will literally be knee-deep in chips).
Granberg also makes a small log mill that only attaches to the bar next to the power head. I have not used it, so do not know how difficult it would be to keep the slab at even thickness. Northern Tool carries these.
You will need a good quality saw to use this. I would recommend either a Stihl medium to professional grade, or comparable Husqvarna. I would also recommend you get a couple of spare chains, as they will dull rather quickly due to the amount of actual wood they are cutting. Changing out a spare is quicker than resharpening, and a dull chain will really tax both you and the saw. I have cut 20" logs with my Stihl 310, but it is very slow going. Worked okay on 16" and smaller. I would recommend a 440 or larger if doing anything wider. When I slabbed some 30" pines, I used a Stihl 660 and it took about 10-15 minutes per cut on a 12' log.
If you are cutting wide logs, a semi or full skip-toothed chain will help to keep the rpms up for better cutting and less chance of overheating the power head. Stihl has a wide range of chain styles, but depending on your dealer, you may have to order them.
Be aware that the demo videos on You-Tube show them cutting straight douglas fir, etc. It is a bit more difficult cutting oak, walnut, or sweet gum, both in the initial set-up to align your cuts to the center of the heart, and the actual speed in which you will cut through the log.
Hope this helps.