Kinda caveman-ish, I dont think its the best, but I had it close by so I used a cold rolled rod close to the the axle size, staight thru both outer bearings and did some measuring in the middle to check overall, and a nice straight edge along the tubes to check those
I just want to ensure that I did'n warp the housing when I removed the old brackets. So a simple straight edge will do?
You can check flanges for parallelism with a digital level. Set the housing with carrier flange parallel to the floor. Measure angle on both axle flanges. Turn housing 90*, measure angle on both axle flanges. The digital levels are quite accurate and measure down to seconds of arc. Did my home-narrowed nine inch for the 31 roadster project this way and it's more accurate than the shop-done nine inch on my 32. If you end up buying a digital level get the 2' model and if you need longer tape it to a longer bubble level. I have a 4' digital and will probably cut it down one of these days.