Update: The gas block arrived today.
Good news - It fits well over the barrel. And having measured it, I will definitely need the mid-length HG.
Bad - News - The shipment arrived with missing parts (allen key and the two set screws were missing). And, apparently, Optics Planet is not taking phone calls right now. Customer returns are also delayed.
Better news - After speaking with both Optics Planet and the manufacturer GunTech, Both have indicated they are willing to supply the missing allen key and set screws. Could be a week to receive them.
So, again, we'll see what happens.