[68RT]

Is this a "cllunk" that is once or every revolution of the engine? Every revolution leads to a faulty gear which could be the first/reverse slider or the cluster gear. Once when getting on or off the throttle would more likely be bad u-joint or excess slack in the differential side gears.

ED: sp

 

[68RT]

Sounds like you either have a pitted tooth or a partially broken tooth.

 

[68RT]

Normally all gears can be inspected by pulling off a cover. Look towards the bottom of this ebay listing for the exploded view and you can see the side cover is pulled. It is easy to do (a few capscrews and lift off) and then all gears can be looked at without a teardown. It can be done without pulling the transmission even.

http://www.ebay.com/itm/NOS-1940-56-DODGE-PLYMOUTH-TRANSMISSION-PINION-MAIN-SHAFT-MOPAR-853488-TRANS-/180977111660