My calculations figured my redline to be at approximately 78 MPH in 2nd gear...

Does this sound about right?

You could use any one of a number of calculators online. Here's just the first one that I have found:
http://davewin.com/tech/gear_ratios.shtml
You have a final drive of 3.42 (GU6) and the GM 4L60E has ratios as follows 1st=3.06:1, 2nd=1.75:1, 3rd=1.00:1, and 4th=0.70:1. You can use an online tire size calculator to get your tire height, but a 235/70R15 is ~28" and a 235/75R15 is ~29". Plug those into the calculator with your redline and you're set.

Here's another calculator that gives you just about everything on one page:
http://www.speedworldmotorplex.com/calc.htm