I checked all my old paperwork and indeed I am using that exact same AllStar bracket #54224 as you have. So to try to better help, I went out to the garage and took a couple better pics. I've got a side shot and an overhead shot (don't mind those blue powder-coated carb parts - it was a used carb!). As far as difficulty in getting the cable into the slot and sawing it open wider, I'm not sure what you mean. I just used a little screwdriver to pry it open a tad to get the cable started into the slot and then pulled it all the way in. Then I slid the metal casing into the hole from the back and positioned it where I needed it to get full 100% throttle and locked it down.
Here's the side shot. You can see where I have the cable positioned in the rear holder and where it attaches to the carb arm. I took the bushing that comes with the bracket, drilled a hole in it, and put my throttle cable stud through the bushing. A nut on the back side secures it all. Note I bent the front part of the bracket down to give a bit more clearance to my upper rad hose. I also tweaked one of the 2 springs to reduce the load. And that was it...it took some layout work and head-scratchin' but I've been happy with it since I installed it - have not had to touch it at all.
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Here's an overhead shot.
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Hope this helps and doesn't add confusion!!