Could be your 1/2 syncro hub, mine did exactly this but with 2nd gear. I've used pattern gears and even a layshaft in my suffix A box and been fine for the last 8 years or so, much better to strip it all down and assess what needs sorting.
Personally I use RTV with great success. I had to swap the T seals on my 2.5 in situ a few years ago and just pumped with RTV instead (cant remember which I used but was a loctite one in a mastic tube I borrowed), much less fiddly than the cork...
Not much of a useful reply but I did it using 2 or 3 small screwdrivers and a bit of swearing, once its started on the shaft a bit of tube to knock it down into place