I bought a cheap ($33) Bandfile from Harbor Freight. It's a 1/2" wide belt sander. I used it to quickly cut down the steel.
I made the block from a piece of 1/2" x 1" bar stock 1-7/8" long. I carefully ground the ends square so I could check both the size of the opening and, to make sure the ends were parallel and square to the sides.
The Bandfile really helped clean up the sides since it's hard to hold them and do much filing.
When the gauge fits flush to the surface the gap is wide enough for the manifold to fit.