DUDE ITS YOUR SYNCHROS, THEY ARE TOAST!!!!!!!!!!!!
Happened to me same problem with it not staying in gear.
your synchros are so worn they wont hold the gear. if you decide to repair the tranny, you might want to check your spider gears too, if you synchros are gone, good chance your spider gears are toast. also one more thing, if you repair your tranny check your shift fork make sure its not bent.

My best advice, get a new tranny off another b series motor. repairing trannys are a BITC* and if you take it to a shop, youll pay more for fixing it then you would getting a new/used one.

well hope that helps. next time, dont downshift sooo much. i know its fun but unless you need to, dont.