Hi All,
Regarding the left and right threading issue, I had a similar problem cutting a LH Acme 10 tpi thread with a right hand tip and tool, completely shagged the work and tip (including the shim).
So I bought some Internal tips and found a Left hand threading tool for said 16Ex tips, the threading tools I ended up with have shim adjusting screws, to alter the helix angle (Didn't use that feature though).
Anyway the job went fairly well after that, as the pic below will show, the top screw is my incarnation of the original lower one(original is 9/16" Acme 5 tpi x 2start, new one 5/8" Acme 10 tpi x 1start).