I'm in the process of making a vertical head for my little Atlas horizontal mill. Much smaller machine, though. I toyed with the idea of using the overarm support rod, 1.25 IIRC, but decided to use a 3" square aluminum block about 6-7" long instead. A plate has a boss that fits in the overarm bore, and a big bolt draws the plate in tight. The spindle head is secured to the plate with SHCS. Right now I'm stuck until my SB9A is running. One thing I will do is flycut the headstock of the mill flat where the plate mounts, and drilling a hole for a pin to help prevent the head from twisting.
Note that even your machine would be rather light duty as a vertical mill, if you rely solely on clamping on the overarm support bar. Maybe an X1 or X2 sized head from LMS could be adapted to your mill. R8 or MT spindle.
Maybe a suitable sized pipe flange as a starting point, build out from there.