The is what I was thinking might be the case.
Did you get 8 lug plates off the burb? also full float axle right? (as they are 3.5" wide).
Makes sense that the piston needs to be in the center of the shoe.
Is your K2500 6 lug? 13x2.5"?
Side note: I changed my axle in Burb with one from a K2500 and it fit fine. This tells me the spring perches are the same, but is the 8 lug axle wider from the perch to the lug flange then? What Im getting at is where is the difference that the backing plates need different off sets?
The axle on the Burb was an 8 lug full float. In retrospect, I wish I had got the full axle housing. The gears and axle shafts were already gone when I first found the Burb, meaning if I used it I would have had to have somebody take the gears out of my axle and swap them into the Burb housing. A short time later when I went back to the pick-n-pull the Burb was still there, but the axle housing was gone.
My pickup is an 8 lug 8600 GVW. I don't know for sure where the difference in axle housing length was at. I had done some measuring and as I remember it the spring perches were close between the Burb and my pickup. However, because of having the pumpkin in the way of accurate measuring I wasn't absolutely convinced at the time that the axle housing would be a drop in replacement. Hindsight is 20/20, right?
Going by my experience and yours I can only conclude that the difference between the two housings would be between the spring perch and backing plate mounting surface. I wish I knew if the axle shafts were different between the two. That would add more info about where to look for the actual difference.
I know the hub is different between the two axles. Brake drum inside the rim mounting flange vs brake drum outside the rim mounting flange. Maybe the difference is made up in how the hub is laid out. I vaguely remember trying the Burb hub on my axle spindle and it not working. Seems like the Burb hub went on too far and the back of the hub hit somewhere in the backing plate area before the inner wheel bearing came to rest in the proper location. At the time I was disappointed enough that I didn't spend any extra time checking out any possible solutions to being able to use the Burbs' brakes.
Since I already have the complete 3.5" brake assembly, drums turned and new shoes, I've been spending some time pondering how I might be able to utilize them with my axle. I'm thinking I might be able to use my pickups current hub, remove the 2.5" drum, and place the 3.5" drum over the outside. Then use the Burbs' backing plate and have a spacer machined to put between the axle mounting flange and the Burbs' backing plate. When I figure I have the time I'm just going to have to tear apart my pickup to see what can be done.
The biggest problem is that we are in our rainy, nasty wind, winter weather now and I'd have to do this out in the weather. I grew up doing farm work after school and weekends no matter what the weather was like and I have a hard time with choosing to work out in the winter weather. I've been collecting parts for one of these 10' X 20' metal frame canopies like you find at Costco, and am almost able to put it together now, so that I would at least have a dry, even if not so warm, place to work in.
Please keep me posted on what you find with your brake project, and let me know of any good or bad things you see about my possible project. Maybe my limited experience may be of some use to you.
Don
ps: I wish we lived close to each other. I'd enjoy being able to see what you are doing, pick your brain, and even lend a hand helping. I love doing these kind of projects where you do something a bit out of the ordinary, have to do a bit of research, and figure out how to make things work. That's why I spent fifteen years installing cabinets, the last few years working in some very fancy and expensive homes with lots of specialty cabinets and wood trim.
