The picture is exactly What I wanted to do! Thanks for the help.
Anytime! Glad to be able to help out.
As far as I know, it's a pretty standard technique for mounting blocks to a boom. I'd recommend narrower and more closely spaced slots than are shown in the photo to get a finer adjustment capability. I have a Bryant extrusion (I think) with about 1/4" slots milled on about 1/2" centers that seems better than the setup I had when that photo was taken.
Cheers,
Geoff S.