hmmm pretty sure i can read... the speed sensor will not be out. the the ECUs in question are both from XR6s both have 3.45:1 gears.

speed sensor will be the same in this case

If the speed sender is matched to the diff then no problems with guages being out.

If the diff ratio is not matched to the ecu, and the ecu uses vss (say in shifting auto's), then it might be a problem.

If the ecu doesnt care about vss (which i assume 5 speeds dont) then again no problem with diff ratios.

Thats how i see it anyway, but yeah if they are all 3.45s its not an issue.