The old Powerglide in my 64 is staring to leak around the seals. It’s got over 220k miles on it, not too bad. A good trans shop can rebuild it for $2500. Or for a few hundred more I could put in a newer Hydramatic with overdrive and the improved as mileage mat help pay for itself.
At least you know it’s going, and can plan for it. I had a bad casting let go with absolutely no warning. Of course, that $3K would be $6K today!
I have the original Powerglide in my 1958 Impala. I've had it rebuilt for your price above and it runs great, but still leaks somewhat. I'd rather have a Turboglide since it has 3 gears but won't replace an original tranny. All my power train numbers match.
I'd go with the Hydramatic to get away from the Powerglide higher rpm's at speed and lousy mileage. Interestingly, the Powerglide was popular with the early dragsters with their high torque and the run was short. But then, they "blew a lot of trannys". Kinda like a Frisco bathhouse. Ha!