Followup on the IHC Model S Speed Truck tire change: Turns out that between clincher tires and split rims there was a (very) brief period of solid rim, tubed tires with retaining rings. Precursor to both split rims and widow makers. Retaining rings are not a concern -- they come off easily and without danger...but the lack of a split rim makes my master plan null and void.
Thankfully there's a tire shop here that knows exactly what this is and can take it from here.
I won't get into just how much swearing was involved when I pulled the tire off and saw that it wasn't a split rim. 90 minutes of trying to use the wrong tools to _attempt_ to get the old tire off of the rim before I gave up in disgust.