Filling/flushing it through the extension housing will only push the sediment laying in the bottom of the casings out onto the sump. Not a lot happens from the shifter mount. I would have a go at spraying whatever cleaner solution up into the trans case with the sump out of the way. Really hit the gear clusters and synchros etc and then use a blow gun off an air compressor to clear it out (unfortunately a nasty job that needs to be done in a dirt free environment so you don't inadvertently blast debris off the floor into the trans).

You should count yourself lucky. There is literally no trim in the poverty spec Gen 1, the Sport gets a nicer instrument cluster and seats with body decals and wheels, while the Royal had full plastic cab trim and carpet + fabric door panels (and the all illusive digital clock). A/C? Pfft, only a dealership fitted option and uncommon at that. Fitting it as an afterthought is tricky as there's all these special ducts and joiners needed to integrate it into the heater/blower unit. A Gen 2 should be a piece of cake to adapt to A/C as it's a single module between the blower unit and the heater box.