In the process of pulling my engine and trans ('68 Chevelle,307, Powerglide trans)I had to dump some of the fluid in the torque converter.
How much fluid should I put back into it prior to installation, and is there any special trick to doing this?

There's nothing special you need to do. The converter goes back in place without the fluid. After the transmission is back in place, with the engine, the trans pump will fill the converter again. You just need to add extra fluid to the trans through the normal trans fill tube.. I always replace the transmission front seal before I re-install a torque converter.