Frank: I don't care what rear gear ratio you have (and I HIGHLY doubt it's all that deep) you should be able to go over 50 MPH. Let's start with a couple of basics. First, if you truly had that deep a rear end your engine would be screaming! Let's assume it's not. Is there a possibility that your throttle linkage is not allowing the carb to open past a part throttle? This could be caused by a whole bunch of different things...the linkage hitting the bottom of the air cleaner, the linkage hitting the firewall, the linkage being so far out of adjustment that when the peddle hits the floor the carb is only partially open. Can you rev the motor up fairly high in neutral? Next, is it possible the transmission is not shifting into high gear? This could be caused by any number of things. A turbo 350 shifts twice....first to second around 10 mph, and second to third in the low 20 mph range (under normal throttle pressure, not being floored). As a last resort, borrow or buy a cheap tach and you don't even have to mount it. Just wire it and set it on the seat. Now you can see the engine RPM's as opposed to speed. You shouldn't be turning over about 2,000 at 50 MPH if the transmission has shifted into 3rd gear. If you really are in the upper 4K to almost 5K range and only going 50 the transmission is the problem.
Back to the rear gears for a second.... the rear end that came with your 283 Powerglide should have been a 3.06. It's possible that someone else changed it, but even so a 4.10 would still take the car to 100 mph at 5,000 RPM. As someone else mentioned, you can get a rough idea by jacking up the car, mark the drive shaft or U-joint with tape, mark the tire and make TWO full revolutions of the tire. I know this sounds strange, but I've been told by two different sources and watched a video and you must turn the tire two times, not once because when only one wheel is turning is actually "halves" the ratio. I proved it on my own car. Did it several times to be certain and it works. You should get about 3 and a fraction revolutions of the drive shaft to two rotations of the wheel. This would give you somewhere in the low 3 to 1 ratio.
Good luck. Hope you find the problem.