I put in a 2004R from a '86 Monte SS in my 72. Paired it up with a 3:31 rear gear (my car is a cruiser and low highway RPM's were my goal). At 70 mph my car cruises around 2100 RPMS. A 3:42 or a 3:55 would work good with a 2004r as well.
And by the way if anyone incorrectly tells you that a 2004R can't handle a lot of horsepower, show them this video. I had an egghead at Jeg's call my transmission "a grenade" when I called to order parts one time (been shopping at Summit ever since).
https://youtu.be/_DDswtLUhdM