So we swapped the rear end to 3:45s and we are still hitting 70 mph around 4k rpm do you think 3:08s will work for power and top speed

 

Lemme guess....you didn't change the gear in the trans to recalibrate the speedo? If you didn't, you'll still be (think you are) "going" 70 mph at 4k rpm's even if you had 2.56 gears.

 

No speedo broken side by side with another car as well as gps

 

then thats most likely the problem. get the speedo fixed. if the trans does not know how fast your going it does not want to shift. to it your going 0.

 

^ I thought that for a moment too. But he has an 82 with a 383 and T10 trans, which would also mean cable driven speedo.
There's absolutely no way your speed to rpm ratio would be the same after a gear swap.
Tell your tire height, and I'll let you know what it should be doing.

 

dang in the wrong thread. id delete it but then you would look silly, please excuse my hasty post. lol. happy holiday.

 

Our 82 trans am has 3.23 gears, and runs right around 3k @ 70mph. Turbo 350, 14" 225 70 14's.