Where can I purchase a plug for my automatic turbo 400 transmissions removal so I don't enmpty that nasty red fluid all over my floor-driveway when I remove the driveshaft ?

 If all else fails, just get a donor yolk for it! When I worked with GM, I got a plug, which I used these last 2 days on separate occasions, from a new replacement transfer case. This fits the large output. I have them for the small output as well. The General, in my opinion, didn't need them back!!! Try your local dealer and ask the boys in the back shop if they have one. Or, ask C1. I sure he has a spare!!!

Stupid me ! I'll just use the one off my 70 Bonneville parts car...Thanks !

use the one out of your Bonneville 

princess auto
 sells a plastic plug that fits right in - i think i paid like $4.00 for it

Bought one at Princess, it is very usefull for quick no mess trany pull out.

Princess Auto has discontinued the plug, so if you see one grab it. 

saw some @ canadian tire last nite - $10.00 little bit fancier package - but look identical

Back in the day when I was doing lots of tranny r/r jobs I made a 2" high dipstick that was brazed closed also a T350 kickdown cable mount piece that was sealed shut.. Along w/ the blank yoke these things made tranny jobs a clean in & outs...both are probably still somewhere in the toolbox but I haven't seen them in years?.

I used to put a large plastic sandwich bag over the rear of the trans and held it in place with rubber bands. Not fancy but it worked.