I haven't messed with a cut AGD stove pipe, but I would expect a shim for the that feedneck. The 7/8" Lapcos fit Luke's vert feed directly, but you need a shim to use the Ego neck properly.
That said, shims aren't bad. It's a small strip of brass that is almost invisible (0.014" thick iirc). It provides some conformal crush between the stainless and the aircraft aluminum, and the grip is very good.
Thanks for the advice! I was nervous about the shim making my rt look kind of hokey, but now it doesn't seem so bad.