What's the power band on your 383 HEEP?
1685 is slightly higher than what 4th gens cruise at stock because we have basically 26" tires stock with 3.42 gears.
Here's the tires I had on my Formula last
http://www.tirerack.com/tires/tires....omCompare1=yes
Look at your power band and see if you have any torque below 2000 rpms. I'm not saying you want have to jump down to 4th to accelerate good while at 70mph, but I can tell you bone stock my formula pulled just fine at 70mph while in 6th gear. I rarely had to come out of 6th on the interstate or back roads when just cruising. Car weighted in around 3500- 3600lbs.
When modifying a C4 or C5 that was an M6, I always went to 3.90s which made 6th more lively but by no means was 6th bad before. It was and should only be used for interstate cruising. Only look at gears 1-4 for performance.