i originally hit up my local lbs about the project and the mechanic gave me a load of bull****, so i decided to do it myself after the reminder of why i learned how to wrench my own bikes. the tool i used was a borrowed
park tools bfs-1.
i'm sure the park tool works similar to what you are using, and i also found that the shell was larger (in addition to not having threads) than the thread guid blocks on the tool. i wrapped the guides in a couple/few layers of painters tape and then tapped them into the ends of the frame. definitely a hack, but it worked. you don't need the guides locked in, but rather tight to ensure an even cut. i don't have the tool anymore, otherwise i would take pictures for you.
this was my first time using that tool and my only other guidance would be to go slow and don't tighten down too much. it'll take a long time to get through the material because the tool is really only meant for shaving off micro millimeters of material. it's like you're using a straight razor to whittle down a 2x4--it's gonna take a while and if you try to go too hard you'll ruin the blade. i didn't use the right type of cutting oil, but i would def recommend some sort of cutting oil.