Why not look into a Gear Vendors overdrive? They're not cheap but you end up with double the number of gears you had to begin with. If you've got a TH 400 you'll end up with six gears. You can keep the 4.11s and still have a nice comfortable cruising speed.
Here's the website:
http://www.gearvendors.com/
69GPModJ
Moderation in everything--including moderation. WC Fields.