Hank, really take your time. read thru all of the instructions first before touching your barrel. then test fit your barrel a few times and see where it touches, dont fit and what it needs to be worked. then take your time with any file work you may end up doing. its not that tough really , a bit nerve wracking at first but its really something that can be done well with a bit of patients. my ml barrel needed the latch notch filed down some and the bottom of the lug filed back, now it locks up and snaps closed tightly.
i just and fired a round off and its great. just me personally, but i dont think i would fit a barrel that needed shimming , i'd fit it to different recievers to see what was close before a shim job, but thats just me..
anyhow, good luck with yours
glenn