TMZ obtained these messages sent between Coppola and then-Paramount Studios honcho Evans way back in 1983 ... and the vitriol explains why it wasn't until last night that FFC buried the hatchet.

The famed filmmaker celebrated the 50th anniversary of "The Godfather" last night with Al Pacino and Robert De Niro by his side -- and, for the first time in public, thanked Evans for shepherding the film 'Godfather' at Paramount.

Their feud began after the original's 1972 release, but 11 years later they were still raging at each other -- and Francis fired off this telegram, saying things like, "Your stupid blabbing about cutting The Godfather comes back to me and angers me for its ridiculous pomposity."

He added, "You did nothing on The Godfather other than annoy me and slow it down ... You have double crossed me for the last time. If you want a PR war or any kind of war, no one is better at it than me."

The battle was, at least partly, about creative credit and Evans didn't hesitate to respond in kind, saying ... "I cannot imagine what prompted this venomous diatribe. I am both annoyed and exacerbated by your fallacious accusations, when all I do is praise your extraordinary talents as a filmmaker."

And, Evans really got personal with this ... "I can not conceive what motivated your malicious thoughts, but if they are a reflection of your hostility, I bear great sympathy and concern for your apparent paranoid-schizophrenic behavior. However, dear Francis, do not mistake my kindness for weakness."

As for how these telegrams are coming to light ... hyper-successful celeb real estate titan Kurt Rapaport was close with Evans, and before his 2019 death, he gave a bunch of memorabilia to Kurt.

Talk about hidden treasures -- the telegrams were inside one of the boxes of Evans' stuff.

While it's unclear if Evans and Coppola ever patched things up, privately -- the fact is, FFC never publicly gave him his flowers, so it was a huge moment of peacemaking.