That's exactly what I suggested but with a collet. Yes, the endmill holder approach is preferable, it also provides the flange for the spacers to rest on!An additional option.......
Most of the arbors i see are too long. Think you need about 8" from the shoulder at the taper end to the start of the threads for the nut.
Purchase an arbor (1") that is long. Cut off the tapered end so you have as straight shaft with the end bearing and threads for the nut along with spacers.
Purchase a good quality 1" CAT 40/BT 40 end mill holder. Then fit the straight arbor into the EM holder add a compromise draw bar stud and you are good to go....You can make the finished
arbor any length you need.
Might need to replace or machine the bushing in your over arm support to fit the inch std. arbor...all pretty easy.
68078 40CV-1.0EM-3.0-PRO. CAT40 SOLID END MILL TOOL HOLDER. Manufacturer: COLLIS. Condition: Used. YOU ARE PURCHASING THE PICTURED. SIZE & LABELING.www.ebay.com
The over arm support I wouldn't modify if I had the intention of finding Deckel arbors eventually. Turning the arbor down to 17 mm and keeping it lubed will last a lifetime.