I found a new method that works pretty well for me.  I got tired of using the silly, messy pump that I have and I can never find a high enough spot to make the squirt bottle types work.  I had an old washing machine fill hose, the kind with two female ends, laying around.  I cut off one of the fittings and that end fits perfectly inside the tranny fill hole, with just enough friction to keep it from popping out.   I left the other female fitting on and it worked great as a small funnel for the squirt bottle type of gear oil containers.  You could also cut off that end and use a real funnel if that works better.   Now I only need that dang pump to get the old lube out of the diff.  

Getting inventive is what it's all about! Keep your engineer brain on high alert

The trans isn't too bad because it comes with a drain.  The rear, on the other hand, ain't so easy.  My '73 had 85K miles and a non-original differential.  So I figured that the current rear has probably been run for about 50K miles.  It was leaking out of every possible place and the half shafts had lots of end play.  Where am I going with all this you ask?  Even if I can possibly drive this car another 50K miles, the rear will probably need fixing and THAT'S when I'll change the oil 

Good luck with that!  Mine's already been done - a ring gear bolt ended up backing out and banged around inside there for a few minutes and then got shot out the back.   I think I had the half shaft yokes replaced at the same time.  Once you get that guy rebuilt, you will want to change the oil on a regular basis.   The only way to get the old lube out is to pump it out with one of those pumps you get an auto parts store.  Mine has never worked very well - very hard to get it primed and it's always such a mess.  Getting diff lube out is the ONLY thing I will use it for now.