Arctic, you're fast! Your material wasn't up yet as I wrote, so take any duplication as a meeting of minds!
Lift, the TM34-B120 4-stroke flat slide pumpers aren't available in the USA; like many other neat things, Mikuni America doesn't see fit to bring them in. But you can get a pair kitted for the XS650 from Topham Mikuni in Germany:
http://www.mikuni-topham.de. You'll be into four figures. They're the same design group as the RS carbs and come with custom brackets, throttle shafts, and choke rods already done, and with the accelerator pump reservoirs linked to run off a single pump. They're done right; bell crank outboard on the left, AP actuator outboard on the right.
A less costly solution, as CDNTX says, would be to find a used bank of Mikuni RS34's (means Radial Smoothbore, not Round Slide, they're 4 stroke flat slide pumpers, only sold in banks of four). You'd need to make a custom throttle shaft and brackets or have them made for you. To avoid interference with the frame, mount the bell crank outboard on the left and the accelerator pump actuator outboard on the right (one pump drives the whole bank of four in the RS carbies).
But you asked about the price-is-no-object best, and IMHO that would be a pair of Keihin FCR 4-stroke flat slide pumpers. Keihin spares no expense: throttle shafts are supported on needle bearings, the slides run on nylon rollers, and mixture control is precise. Again, you're into 4 figures; Sudco can build you a custom set if you want it, and you're guaranteed to have the only one in town!
Re. Dellorto, it's an old round slide design, and as far as I can make out there's very little you can do about timing the AP for turn on and turn off. IMO the Dells are overpriced. No jetting data pool that I know of.
A little searching here and at XS650 Garage USA will turn up a lot of jetting guidance for Mikuni flat slide pumpers. A word to the wise: the difference between 34 and 36 mm. Mikuni carbs is going to be fractions of a HP in favor of the 34 down low, and fractions of a HP in favor of the 36 up high. If the carbs are decently tuned, you'll never know the difference on the street. I use a pair of TM36/68 flat slide pumpers. Like all TM 4 stroke pumpers they were set up as separate carbs for singles. Had to disable one AP, link AP reservoirs, link the throttle shafts, drill and tap the unmachined bottom bracket bosses and fit a bracket, open the choke rod holes a tad for a BS34 rod, and modify the tops of the choke plungers by removing the knobs and grooving them for modified BS34 actuator forks. Jetting was a breeze--component effects are way better isolated than in the old stuff.
