I fly my Big Bertha (a survivor of my first rocketry days) on C6-5 or B6-4 motors. It's great on either. The flight is just the right level of stability. It rides up like it's on rails, and it doesn't tend to weathercock overly much.
Doing some quick and dirty measurements, my Big Bertha is approximately 23 1/2" tall with a center of gravity as follows.
Empty: 11" from the nose tip
Loaded with a B6-4: 12 3/8" from the nose tip
Loaded with a C6-5: 12 3/4" from the nose tip
If you decide to go with a mount for D motors (24mm), just install it as you would for a mount for A-B-C- motors (18mm), placing the rear end of the mount in the same position as the stock directions specify. [On mine, the rear of the motor mount tube is flush with the rear of the body tube, and the rear centering ring is recessed about 1/4".] Then add nose weight (if necessary) to bring the center of gravity back up to about 12 1/2" from the nose. The Bertha has plenty of extra room in the body tube for the extra thickness (and length? - See below.) of the 24mm motor mount. You'll need new or modified centering rings, of course.
While you're at it, why not build it to the length for E motors (assming you have the space to fly a Bertha on an E or the eyes to track it)? Then cut a short piece of a spent 24mm motor as an E-to-D adapter (Just slip it into the motor mount before the D motor.), and make or buy a 24mm-to-18mm adapter. Then you can fly that baby on anything from a B to an E! I love rockets with wide motor ranges.
Be careful to weigh the rocket when finished to make sure the B6 can still loft it safely, and pay special attention to the fin attachments if you want to fly Es. Glue rivets come to mind, and proper tube preparation is important. Do a double glue joint. If you're really ambitious (for a Bertha, that is), cut your own fins and include tabs for through-the-wall attachment.
Micro: Will a Bertha fly high enough to recover properly on an A8? I can see it better with a pair of As.
