Just because it looks like junk,doesn't mean it is...
We sold quite a few manual trannies & T-cases out of aincient cars and trucks at the junkyard,that sat a long time,some out to the weather just like this setup pictured--most of them ,if they were not seized up tight,ended up "living" --if they turned by hand,chances were good they were saveable..after they were drained completely and flushed out with some diesel fuel,we used the parts washer after taking the side cover off..
Most of the ones we sold,before selling them,we rigged up a v-pulley to the input shaft and filled the tranny with diesel fuel,and some 15W-40 diesel oil (high detergent) and used an electric motor to spin the shaft (we used an old table saw we got in the the scrap pile!)--after letting it spin a few minutes in neutral,we'd shift it into all the gears and let it spin awhile...then dumped it out and blew the gunk out with the air gun--and told the customer to use fresh gear oil in them--some added some Lucas or STP gear treatment to the lube too,claimed it made them shift easier and run quieter..
Only a few of them ended up being noisy,or not shifting well (like the synchros were junk--might have been to begin with?)...we were often amazed at how well a lot of gearboxes and diffs survived after sitting a long time out to the weather,yet still ended up being perfectly useable as-is...a few didn't pan out though,some had the cases crack after water that collected in them from condensation or rain,froze solid and ruined them...we used some for parts to make one out of two or three for a few of the antique cars we restored though...