I deleted the heater core and was looking for solutions on coolant plugs for the rear housings that won't let go anytime soon.. I tried a cap and caught it before it split over from dry rot.

 

This might help. I know it's 2nd gen however it may give you some ideas.

Suggestions to block off coolant ports (s5 TII)? (turbo coolant feed/heater core)

 

Because this location is directly below the Oil Filter Pedestal, and when those O-rings start to leak, the dripping oil results in softening of any rubber hoses, I used a 3" section of heavy wall coolant hose with a hose clamp on the iron side, a long (~2") rubber stopper shoved in the end, and a hose clamp around that to hold it in place. It ain't pretty, but it has never leaked, either.

Alternately, you could look at one of those radiator flush kits which include a garden hose fitting and see if that could be made to work. I wouldn't entirely trust that plastic, though, as it can get brittle with heat cycling.

Don't forget to plug your radiator return. It's in the lower hose fitting. I JB-welded a plastic plug in it, then a rubber hose cap, and a hose clamp for good measure.