I had a wide ratio T-10 in my '64 Skylark. Put a Gear Vendors behind it because everyone drives 80MPH in the slow lane...
The T-10 trashed third gear and the cluster, time and money to get a Richmond Super T-10 (3 months minimum) I went with a Tremec TKO500.
I had to cut and raise the tunnel and that affected the carpet lay, so I ordered the carpet "untrimmed".
Now Tremec has the TKX, which is designed to fit A-Bodies without having to modify the tunnel.
If you like a pedal car, look into that as an option.
https://www.tremec.com/menu.php?m=183There are several places that sell "kits", American Powertrain, Modern Driveline etc. (I purchased mine from Modern Driveline).
Now, I can run 80-90 at about 2800 RPM, and get great mileage!
Hope that helps.