Depends on your goals, no question that 3.90's will buzz on the highway but bear in mind that a RAIV engine seems less annoying because of its free breathing characteristics but also NEEDS steep gears due to the high valve overlap-duration (no bottom end grunt), remember that the 3.90's were a compromise to begin with in 69, all previous Ram Air and Ram Air 2 cars came with 4.33's only. I think 3.23's would be a mistake for a RA IV car and 3.55's would be close enough that you might as well have the 3.90's. My opinions though are based on the typical collector car lifestyle of mostly sitting in a garage with limited cruising. If you wanted to use the car for long drives and trips maybe a overdrive automatic (TH700 etc) is an option.
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My Break Away Squad
1969 Fbird (Base, 350 & Sprint Cvt’s - 400HO & TA Hardtops)
1969 LeMans (2dr & 4dr Hardtop and a Cvt)
1969 LeMans Safari 2 seat Wagon
1969 GTO (2 Cvt, 2 Hardtops & Judge Hardtop)
1969 Catalina (3 Cvt’s & a 2dr hardtop)
1969 Ventura 2 Seat Wagon
1969 Executive 4dr Sedan
1969 Bonnie Cvt
1969 Bonnie 3 Seat Wagon (2 of them)
1969 Bonnie Brougham (4dr Hardtop & Cvt)
1969 Grand Prix SJ (2 of them)
1969 2+2 2dr Hardtop (Canadian model)
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