My read of this situation is that you have a reputable company (or two) that makes conversion parts and one of their parts, for whatever reason - be it an initial measurements issue, measurements to prototype fabrication issue, prototype to production piece issue, whatever - has a motor mount that is about 1" too short to be a "bolt in" piece of hardware.
For what a person pays for this what's supposed to be a "bolt and go" part, I would not pay to have either the motor mount modified, nor the pad have an adapter made for the mount, either. This is twice now that this same manufacturer has had the exact same issue on two different installs with the same part. Whatever the cause, the manufacturer needs to address the issue, not only in the field with customers using the motor mount conversion kit, but also internally to find out why it is having an issue with the DS motor mount being so far off from fitting the application and either way, making it right with the customers who have had this problem.
Otherwise, if the customer possesses the needed skills, or local fabrication can be done at low enough cost, I'd get a refund (or not buy the kit in the first place) and just fabricate up a set of motor mounts on my own, instead.