3MT (2MT on all 6") collets are the only ones that fit directly into the spindle nose. The 3AT, 3C and a few other rarer ones all have a closer adapter that goes into the spindle taper and then the collet goes into it.
OK Robert , yes as usual I didn't do the research . I think Dan has a 12" Atlas , Joel has this set up for sale https://www.mymachineshop.net/product_p/10075.htm
I think the sleeve is the way to go , that's what I believe Dan was asking .