Toolholder material recommendations needed

Hi Group,

I have a heavy duty wood shaper/moulder made in Italy with a vertical spindle that has a female taper a bit larger than a MT#5. The toolholder/arbor is held in with spanner nut. The arbor for the the cutters is 1.25 dia. and about 7" long. I would like to make or have made a custom arbor with an ER20 collet socket on the end so I can mount standard 1/2" shank router bits. The existing arbor taper is not a ground finish and the material files like 4142 Rc 30-34. I read that Parlec uses 8620 for their CAT40 type tooling.
So, does anyone have recommendations as to what material or heat treatment would be best? The arbor does have a nice 1/8" radius where the 1.25 dia. changes to the larger tapered diameter most likely for stress concentration issues.

Thanks in advance,
Pat Dexter