I am I right in the thinking I would just need the one spacer (front left mounting point) to make this work?  Besides removing the grab handles.
I did a bit of searching and that is the only comment I found that seem to go with how the bracket seems to fit.
Although some seem to indicate it should just bolt up.

Anyone been down this road already?

Are you talking about the 35L K1100LT top case? I'd advise against it because the K1100LT rack is plastic so the case bounces around a lot when riding.  Those racks also crack on occasion.

I assume it is the 35L LT top case/trunk. I have heard of them bouncing around and breaking brackets.

My tail fairing is already drilled for the rack and smaller case so it should be easy enough to use that to help stabilize the larger case. Just thinking a couple of little aluminum Z brackets would make a lot of difference. Should be able to come up with something between the existing holes and the rack.
I see you have a couple of LT's on your list. Did you run the bigger trunk on them?

I actually sold one of my LTs a while ago but yes, I do use the 35L top case on them because that's what they came with but I wouldn't put it on any other K with the factory rack for the reasons that I mentioned.  Maybe it's just me but it bugs me when I can see it bouncing around in the mirrors.

A couple of other nits about the 35L top case:
1 - It's heavy
2 - The latching mechanism that closes it is weird. Not a big deal though.

However, it is a nice big top case that holds a large volume.  I've run one on my K1100RS but  not with the factory rack. I bought a butchered bottom half for cheap on Ebay and then made some custom spacers and bolts to attach it to a Reynolds Rack.  The downside of this is that it's bolted on and doesn't pop on and pop off easily so I just leave it on the bike when touring.



 If your tail cowl already has holes in it then you could do something similar with one of these Givi racks made for Ks.  It fits the same holes as the factory tail rack. Givi part is E180 or E181.  The picture shows it with a Givi base plate but you don't need that plastic part except for mounting a Givi case.