I've recently got a 50 3/4 3600 4spd. It's my first experience with these trucks. If your 53 has the same rear as mine, the original gear ratio was 4:56. Based on my research, the 67-72 3/4 (C20) has the same "guts" as ours. If you can find one that had/has an auto transmission, then the ratio should be 4:10. You can pull the whole gear assembly out and bolt it right into your housing, using your original axles. Then you will be able to travel faster than 45mph without blowing it up :-) I found one for $100 and am going to try it out.
Check these out:
http://community-2.webtv.net/Tom3600/REARENDGEAR/
http://www.pirate4x4.com/tech/bible/index.html#GMRE
www.stovebolt.com
Good luck.