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 OHC, 350, 400HO & Sprint Cvt’s - 400HO & TA ht’s)
1969 Custom S 2dr ht
1969 LeMans (2dr & 4dr ht & Cvt)
1969 LeMans Safari wagon
1969 GTO (2 Cvt, 2 ht & Judge ht)
1969 Cat (3 Cvt’s & a 2dr ht)
1969 Ventura wagon 2 seat
1969 Exec 4dr post
1969 2 Bonnie Cvt's & 2 wagons
1969 Bonnie Brougham (4dr ht & Cvt)
1969 Grand Prix 2 SJ’s
1969 2+2 2dr ht (Canadian)
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