syncro replacement?

[jakeblues652]

my third gear syncro is shot, and i was talking to a machiene shop specilizing in transmissions from TX i think, and they told me $20 for the 3rd and 4th gear syncros. i hate having 3rd pop out of gear all the time so i want so replace it. how hard would this be? im not an expert, but ive been turning wrenches a while, and i know lefty loosey righty tighty i've had engines apart and everything, but trannys are a whole new ballfield.
its a 4 speed. im not sure if its the SM420 or SM 465. its from a '67 4x4. and reverse is all the way towards the driver and down.
you think i could tackle this? i dont want to spend the money on having a tranny shop do it, and i dont want to try my luck pulling one from the local junkyard.