Sounds like the seals got dry on the transmission - I would suggest keeping an eye on the level for a while. Don't be surprised if you might wind up replacing some of those seals and gaskets.

Total fluid capacity on a th350 is 10-12 qts. depending upon the converter size that's used. Most trucks will use the 13" converter, so I think that you would be in the 11+ quart range. If you have an auxillary cooler, that usually adds 1/2 to 1 quart to the capacity.

Apologies if you already know this, but always check the fluid with the tranny in Park, at operating temp, and with the engine running. Reason for this is that the converter will drain back into the pan when shut down and give a false (higher) reading. When the car is started the front pump will draw fluid back into the converter and give a true operating pan level. Also, the transmission fluid expands slightly as it heats up, so taking the reading at operating temp is important.