Randy's is pretty good price and good quality for new. If a 3.54 is made for that axle he is going to have it.
http://www.ring-pinion.com/
What transmission are you running? If you have Overdrive I would go with a 3.73. It will help keep the tranny from hunting. I run 3.73 in my 454 th400 burb and my 79 K5 I just sold. Tires are 31 inches tall. 70 mph is just under 3k on the tach. My75 has 4.10's but runs 35's and 70 it's right at 3k. With OD it would kick back to about 2350. Much lower then that and it's going to cause it to hunt with any sort of grade.