Double check how to determine the rear axle ratio. The way in which you did it requires that you multiply the number of turns by 2. I have 63 C20 with a 4.10 rear axle at 65 mph I'm turning 3300 RPM. If your ears are correct your guess of about 3000 RPM is correct, you have the rear axle ratio 4.10.
You can use the same bellhousing with those transmissions.
