71 suspension or use what I got?

[N2TRUX]

Ok, so as far as the rear goes, you don't have a 68 axle to start with. Cool! The 4L60E has a low enough first gear and the 5.3 has decent low end torque, so you might be just fine with a 3.08. I would consider trying it before changing to a 3.73.

That's a great gear if you like smoking tires and grinning a lot. If your after MPG, not so much. The 3.42 is a good compromise and works well with an OD transmission.