fixture a pin on a rotary table mounted flat on the mill table

get a 4 or 6 inch phase 2 type table , relatively priced

clamp your bushing in the vertical orientation with a custom pin and bolt to
make rigid

mill out the 90 degrees using a 3/8 end mill by side milling in and then
rotating the table 90

leave a little to clean up

change to an 1/8 or 3/16 endmill and finish the cut

will leave a very small radius in the corners , good for strength


after initial fixturing , should take less than 5 minutes each in a
Bridgeport



could do it in a shaper

could do it in a cnc mill

don't leave a sharp corner

finish the bore after cutting the notch for best results