It would be a good idea to determine your current gear ratio before you make s decision. The most accurate way is to pull the cover and count the number of teeth on the ring and pinion gears, then divide them into each other. If you choose not to do that look up an online gear ratio calculator. Then enter your desired highway cruising rpm, that will get you where you want to be.
If you are doing say 3,500 at 50 you probably have like a 4.56 / 4.88 or something like that in there. Way too low. If it is an automatic I would also look at the converter because that may be slipping a lot too. I agree with the guy above the 3.08 if you are going to do a lot of highway driving. If you poop around town a lot but want some highway flexibility then maybe like a 3.42 or so. Based on todays highway/driving a 3.0 or lower is prefereable for being comfortable on the highway for long periods.
Transmission has a lot to do with it as well. I assume you have a Powerglide. When I got my '67 I think it had 3.08's with the PG in it and wouldn't get out of it's own way. I switched to a 200-4R and 4.11's which was a good combo. Now with the TKO 600 I think I'm going back to 3.73's or 3.55's as first gear is nearly useless.
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