You wont find much, just research upgrades to the 68rfe and those will be able to be used in your 66 assuming we are talking valve body.
All you want to know is in video, I make no recommendation on this brand either way, probably as good as the rest, but this video found by hemi395 is a great resource.
Clint Cannon, President of ATS, describes the inherent weaknesses of the 68RFE (545RFE, too) valve body. This is a comprehensive overview of the numerous pro...
youtu.be
Look up the
synthetic filter for rfe thread and get metal threads one time purchase cheap and use synthetic filters for canister, only use oem sump filter for rfe. Redline c+ has been quite the good fluid upgrade took away low rpm shudder I had since new. Mixes with oem no issue, drop pan change filters would need 8 quarts.