This is a pretty standard modification and Barrett sells these crossbars with their cold air induction. The dimensions of my setup are in the photo below.
I unfortunately measured once and cut/welded twice. The first time I welded one in, I more or less eyeballed in the dimensions while the engine mount was on the weld table and not on the airframe. After I mounted the engine and allowed the mounts to sag a bit, I was not happy with the clearance I had remaining (1/16"). As such, I removed the engine off of the mount and welded a new cross bar in place. The tubing was pretty easy to find and I just ordered it from: McMaster Link
With the Thunderbolt IO-540 setup with Cold Air, .82" of cross bar drop got me a little over 1/2" of clearance on a mostly settled engine.