Hi Mike
I agree with what Bodrighywood has said.
The only time I've heard of the concerns you mentioned: 'something I was told to avoid since the extra length acts as a lever and increases the strain on the spindle and bearings" related to having a long, weighty piece of timber held in chuck jaws, especially the wrong type of jaws and without tailstock support.
What I neglected to say in previous post is that a chuck, faceplate or insert/exert adaptor should 'mate' with the shoulder of the lathe spindle directly behind the threaded section to ensure true concentricity and alignment. Whatever you thread onto your lathe spindle needs to 'mate' with this shoulder. So as you have an Axminster chuck I'd check what Axminster offer or suggest.
I don't know the exact question you asked their tech dept but I'd have thought they would've at least mentioned an adaptor as an option as well as telling you that changing the backplate was not an option.
I've attached an image which shows the shoulder of the lathe spindle that the chuck, faceplate or adaptor that you screw on needs to 'mate' against.
Hope this helps.