if you have those tyres and are doing 70mph you should have the following rpms (this assumes you have 2.79:1)
2638 in 4th gear
1794 in 5th gear (t5)

if you change that to a 3.5:1
you'll have
3310 in 4th and
2251 in 5th (which is still good)
at 70mph

if I'd take the max speed you could do in the gears at 5000 rpm
with 2.79 gearing:

40 in 1st
67 in 2nd
100 ..
133
195 in 5th (that's all theoretical if you had enough HP)

with 3.5 gearing this will change to:
32 in first
53 in 2nd ..
80 3rd ...
106 4th
156 th

i got this info from a calculator i made myself when I had to decide for new gearing.

So basically if the following is valid: "you can do 70mph in 5th at 1800 rpm", then the following will be valid as well:"2250 rpm in 5th at 70mph"

and all the rest ...

 

If everything was installed correctly, the hesitation just sounds like a lean spot in the pump shot. Pretty common when adding HP.

 

Thanks for all this information, I will check my RPM next time on the freeway.

I posted an update here: https://mustangforums.com/forum/clas...ml#post6060312