Back in 2005 or 2006 I was considering purchasing a used class C with a seamed cabover that got severely damaged from hitting something low-hanging. I had researched RV manufactures at the time that were offering new rigs with a seamless bucket. None of the companies were willing to sell me that bucket nor wanted to share their supplier. I also researched B+ caps getting the same results.
After you exhausted all other avenues, I would call an RV company who sells a motorhome with a bucket you think will work for your rig and tell them you have one of their units in your body shop. You'd appreciate the name of the company that supplies their bucket. I hate lying like that, but I don't know of any other way to get that information. Then call the supplier and spill the beans that you own a different rig with X Y Z dimensional requirements, and ask if they have a bucket that fits your need. If not, ask them who else you could call.
For reference, the pictures below is a rig like ours getting it's B+ cap installed at the factory. The B+ cap is very light weight for one man carries and places it into position, and then screws it down all around to the van roof.
I would seek a C bucket or B+ aero design that follows the lines of and fastens to the perimeter of the van roof like ours does. This to eliminate that major challenge. Then you only have to be concerned of the over-all width & height & transitional side/wing walls. If you own a Ford chassis, you will have many more choices.
I would be very interested in what you learn through your research as this subject had me in deep thought (more like obsession) many years ago.
Note how the B+ cap rests tight (screwed down) to the perimeter of the van roof. It also has a rubber gasket between cap & steel which has a rubber round bead that finishes the joint nicely and good against water infiltration. The cap is well secured to the van roof. That is easy to duplicate in a cab-over conversion. I would look for this kind of cap-to-van joint design to reduce conversion challenges.
Here is a non-bucket example with a joint I would try to avoid as I would not know how to reliably secure a bucket this way.
