Well, first off I would drain the GL-5 lubricant out and put in 75/90. The 833 will not shift right with GL-5. I was just dealing with this on a Hemi Cuda, customer put in GL-5 and it exhibits the same symptoms you describe even through upshifts. Changing the fluid fixed it. Changing the fluid is the easiest, what I would do first. After that, sounds like synchro problem if its only 2nd gear downshift. I bet the fluid change will fix it. I actually ran Dexron ATF in the one I had, it came factory filled with ATF-shifted great. They changed the synchros in '70, prior to that some had SAE 90 as lubricant as factory fill.