66 poly 318 trans behind 340 in 71 Demon.

You can not use a /6 trans behind a V8
The converter should be a neutral balanced unit. No weights. If there is a weight on it, the converter itself may have needed it to be balanced neutral. My only advise to the converter is ether get a new one or have that one checked out for being a neutral balanced converter.
IMO, I *think* a new one will be cheaper and easier.