If you have original equipment.

Rear-end info:
http://chevy.oldcarmanualproject.com/chevyresto/56139.htm

Rear-axle ID:
http://chevy.oldcarmanualproject.com/chevyresto/56139.htm

You'll most likely get a lot of opinions on what to change-to for higher speed. i take a "keep it simple" approach. The 3.38 conversion will get you comfortable 70 mph cruising speed. The SM420 transmission has a rarely used "sub-low" that might become useful with the 3.38 rear.