Inserts are sold. Gutted oem 6th Gen Mustang strut housings with gland nuts and adapert/collars are still available.
One used pair of Koni 8611-1257RACE Double Adjustable Strut Inserts for sale
Prices plus shipping:
Inserts - $250 for two (new $750)
M51 x 1.5 Gland nuts - $25 (new $50)
Adapter/collars for gland nuts - $50 (new $110)
One insert works. The other has a jammed/broken rebound adjuster. Rebuild of the broken unit will cost about $200 per Jeff Wong at ProParts USA during their current sale or for free at the Koni Trailer at the 2017 Solo Nationals.
The adapter/collars are currently welded in F Street legal 6th Gen Mustang struts and will need to be machined off by buyer for other applications. Identical inserts and collars have been transferred from another 6th Gen Mustang into a 6th Gen Camaro.