Hey man, I saw your car in the Bargain News!!
I have 4.10's in my '96 GT. At 70 it runs about 2400-2500. My speedo is a little off, but that is my guess after having people drive next to me and test from there. Both my '89 and old '91 have 3.73's. The '89 is a manual and it runs about 2200 at 70. The '91 is an auto and runs about the same. I think the 3.73's are better for the 5.0 and the 4.10's for the 4.6. Most on the forum agree.