C6 transmission issues 1978

[ccdje]

Newbie here. Recently bought a 78 Diamond Jubilee edition. Seller is very knowledgeable regarding these cars. Car has been sitting for 3(?) years and was recently started. However the trans won't engage. He tells me that it was way overfilled at some point and is now like mud.
Question is; full flush or full rebuild? He did state that when in gear, he revved the engine to 2k and the car did lurch forward.
Any thoughts would be most helpful. Oh and I don't have the car home yet. It'll be shipped sometime in February.

 

[74 Harley]

A C-6 is a very durable transmission, however it sounds like a rebuild is in order. Fortunately it is also relatively simple compared to modern transmissions.

 

[OneAmongOthers]

A C-6 is a very durable transmission, however it sounds like a rebuild is in order. Fortunately it is also relatively simple compared to modern transmissions.

Agreed. A flush on a 40 year old trans is only gonna flush out all the good with the bad. Yank the trans and get it rebuilt.