Check out this site:
http://www.f-body.org/gears/ You will need to know the height of your tires. I usually set my redline at 3500 to figure the max speed I could possible tolerate in any given gear.
I think you could tolerate 70 for short periods, assuming your brakes are up to the stopping task.
Normal around town speeds of 45-55 should become very pleasurable instead of right at the limit of tolerable and your mileage should really improve.
Again, I urge you to make sure your brakes can stop you at higher than originally expected speeds.
