If it's the original converter from 68 it should have a drain plug. Follow the procedure in the service manual to flush it. Take an input shaft and insert it in to the converter and turn the input clockwise and counterclockwise several time to churn up the fluid. It also helps to rotate the converter at a 45 degree angle before doing this. Then drain all the fluid. Install the plug and pour in clean kerosene or mineral spirits and repeat this procedure a few times or as many times as necessary until nothing comes out but clean fluid. Once clean and thouroughly drained install the plug, torque it and add a quart of the trans fluid you are planning on using. Rotate that input shaft a few times and you are good to go. Back in the late 60's and mid 70's during the core return program over 90% of the converters that came back from dealers were pristine inside upon cut-open. That's before we went to lockup converters. The service manual,also has a procedure to check the stator ORC - if you drive this converter and it seemed ok you should be ok but it doesn't hurt to test it anyways. Especially if very fine metallic debris comes out. If this was a lockup and you had water in the fluid then the lockup disc is junk and flushing will not prevent its failure of the disc.