Agree with the points that CEM has made. I would like to elaborate on footbed selection.
If a ski retailer is only invested in one type of footbed, they will tell you that this brand and method of construction is the best. It would not make sense for them to do any other thing. Some have used different types over the years and made a choice as to which they believe in, or which is easiest to build, or which is the most profitable for them. Others have only built what they have known for years. More then likely if a retailer only offers and only knows one type of footbed, every customer regardless of weight, foot shape, arch flexibility, boot model (low or high volume, narrow or wide) gets the same footbed.
Some footbed brands can be easily adapted to match your personal needs, either by how they are cast, or how the foot is positioned in casting, or in the choice of materials used in the cast, post, and finish.
What is the take away of this post? The take away is that you want to get your footbed built by someone that has a good understanding of your foot, your existing problems with footbeds, and someone that will be there to help you get through the break-in period. The other piece of the equation is to get the footbed built by someone that will explain to you what they are doing and why they are doing it. So if they choose to put you in a XYZ brand footbed, make sure the reason for it makes sense to you. If the explanation does not pass the smell test, move on to the bootfitter that does.
For your particular situation, you are a big guy with an arch that sounds as if it does not flex much. As a gross generalization via the internet, it seems to me that you need a device that offers some flexibilty (perhaps softer posting material, or a device that has no posting, or just a heel stabilizer and no post arch). Without a full evaluation this is stepping out on a limb, but you sound like a candidate for a weight bearing or semi weight heat moldable blank that offers posting options.
The best advice is to find a bootfitter that can assess your needs and build a one of a kind device for you that addresses your needs.
On the subject of canting with the footbed(especially with your arch) I would not recomend it.
Get a good accommodative footbed built, have it placed in the boot on a flattened bootboard, have your cuff aligned to match your lower leg coming off the base of support, and then align your knees over your feet.
You can always play with internal tweaks down the road. So get to ground zero first. Comfortable, supported, aligned.