I ordered a 3-7/16" L x 3/4" dia 2 flute center cutting end mill with a 5/8" shank from McMaster Carr, the cheapest one I could find in that size, about $30. BTW, I ordered it yesterday afternoon at 2:45 PM and it arrived via UPS this afternoon at 3:30 PM, along with several other items. Amazing service those guys. Anyway, I couldn't wait to try the thing so I ran down to the basement and chucked it into my Mini Max J/P. That 5/8" shank just fit into the 16 mm Wescott Chuck. Turned it on and made a 1-1/2" deep mortise about 3" long easy peasy, beautifully smooth with no chatter. I only took 1/16" - 1/8" cuts just to be safe. The resultant mortise was as smooth as those I cut with my router and a spiral upcut bit. It measured almost exactly 0.75" which proves there was no chattering going on. The bit stayed tight in the chuck.
My next move is to look for a 5/8" end mill of the same style that will give me a 2-1/2 - 3" cutting depth. That should be sufficient for most passage door applications. If that works OK I will have resolved my issues with what bits to use with my J/P mortiser.
John