Ok guys, question. Stock 283, powerglide, and stock rear diff. Not going for hp or bad assary. Just want to hit 65 mph with ease. I ve heard to change the rear diff. If that's true what are the suggestions that require the least amount of swapping?
I don't have a good answer for your question. But I always thought, if I was upgrading something vintage like yours, I'd want to upgrade to a newer transmission with more gears and a lock-up torque converter, and choose rear-end gearing to go with that.
I'm aware of the 700R4 transmission, but I've seen them criticized for not being durable. Not sure what the truth is, or how much you'd have to spend getting one upgraded, but I'd like to think that mounting one behind the 283 would be fairly straight forward.
As for the rear end, with a powerglide or other GM transmission with a final drive ratio of 1.0 (eg, turbo 350, turbo 400) I would think something in the range 2.75-3.25 would work OK. Seems like GM put lots of 2.73's and 3.08's in b-bodies back in the day.
With an overdrive transmission like the 700R4 (final drive ratio of 0.7), you can put a little bigger numbered gear in the rear, which will improve acceleration while the lower final drive number will help keep the RPM's down at highway speed. The extra (high) gear in the transmission makes up for the bigger numbered rear end ratio.
So, with an overdrive tranny, I'd think a rear end gear ratio in the range 3.0-3.5 would be a good fit without killing gas mileage.
GM has another, newer overdrive transmission in the 4L80, but I've no idea how easy it is to use in a retro application like yours. But that may be something to consider as well.
There are a few guys here who have actually done this, and not just thought about it. Hopefully, some of them will chime in