The McR38 has an opening that just clears the plastic surround of the SSC. To acheive the proper focus while maintaining the relative postitions of the LE, reflector, and head (for proper lens pressure on the reflector and tube ground connection), the SSC should be raised about 0.030". If it's a McR38 Joker, 0.027". But 0.030" works either way and I can't tell any difference.
Since the SSC sits higher in the reflector, the bottom of the reflector can and probably will contact the LED leads, and short out the LE. So when building the Seoul LE, utmost care must be taken to properly center the LED, as well as bend the leads flat against the surround. I also coat the leads with a skim of epoxy as insulation just in case.
I haven't bought a complete SSC LE from the Shoppe, but I'm guessing these issues have been addressed and it will screw right in.
Although not always recommended, I've run an NG750 and NG1000 SSC LE off an RCR123. The LED is overdriven in direct drive until the cell voltage drops below the Vf, but I carried one for a year and had no problems. You could also run a GD board for flatter regulation. For extended useage, I'd go with a 750. Heatsinking is decent but not excellent in an Aleph head, and 1000 mA is pushing it for long runs, but fine for intermittent use.
The GD does not particularly like a 2-stage switch though. You need to keep the resistance at 10 ohms or less, 5 is good. Otherwise it tries too hard to boost the reduced voltage and will flicker. The NG behaves "normally", so if your tail is the standard 15 ohm or greater, the NG is the way to go.