You could go one of four ways here. Let me work them outloud and then I'll give you my opinion on the best bet....
Option 1) a Medium with a flat bar, short stem and the max in spacers (a single 60mm and a single 20mm)
Option 2) a Medium with a rise bar, short stem and two 20mm spacers + three 5mm spacers.
-----keep in mind that those two options result in a Pad Y of 650 - 2mm lower than your number. The next Pad Y is 655 - 3mm higher than your number and then the Medium would have to have the rise bar with a single 60mm spacer under the aerobars. Also, in terms of Pad X - you're right on the edge of this set up: either pads back all the way on the standard mounts (yeilds a Pad X of 489 - 5mm longer then your number or use the TSP and push pads nearly all the way forward on those and I think you can get within 2mm of your 484
Option 3) a Large with a short stem, flat bar and two 20mm spacers + a single 5mm spacer. This gives you the 650 Pad Y.
Options 4) a Large with a short stem, rise bar and a single 20mm spacers - again, 650
......if you wanted to error up on the Pad Y in size Large then it would be 2x20mm spacers + 2x 5mm spacers for flat bar...or...1x20 + 1x5 for rise bar. And, in terms of the cockpit distance if you use the TSP with pads all the way forward you get 484 on the nose. If you forgo the TSP and mount the pads all the way back you'll have a Pad X of 511
Now....which is best...hmmmm... Go with the Large, the flat bar because it comes stock. Add 45mm of spacer so you are on the bigger bike with longer front center. It'll give you some room to move if need be.
Ian Murray http://www.TriathlonTrainingSeries.com
I'm all about athletes mastering triathlon
Twitter - @TriCoachIan