I am not sure I would say that you don't have an X-mag. I am definitely not a Euro X expert but you may want to take a look at this:
http://xmagworld.com/2012/12/22/the-std/
Bunny could probably give the best history.
From what I gather the Euro X could have come with either standard or ULE lowers. I am not exactly sure what the ace harness looked like.
Either way I don't think you could truly convert standard lowers to ULE lowers with out having milling showing, rear hump, area in the trigger guard. If you just want an ACE slot than I get where you are coming from.
Do you have an ACE board to put in it?
Either way I think if you like the way it shoots than I would enjoy it, and if you really want ULE Lowers it may be worth asking if Caustic can match a frame.
Les see a picture of the gun
(PS I have never seen ULE Lowers CAD Files published)