Consider a quill to threadless adapter on top of that tube extender,
[ Which can be ordered thru any LBS, (not just Harris in Mass)
as most if not all distributors that supply bikeshops will have one in their inventory].
and then a currently more common threadless stem can be fitted
onto the top of the Q>TL . reach-length is quick to change
as current threadless stems have removable faceplates
so bars come right out , to swap stems..
brake passes thru current stem, so a cable hanger needs to be purchased to attach to the quill riser,
QBP makes one , also available in most LBS.