did you use the trans. speed sensor that was connected to your old tranny or did you use the sensor that was on the t-bird tranny?also what size rear end gears do you have?
even if you have the correct driven gear on your speedo cable to compensate for a rearend gear change, the speedometer drive gear in the transmission might be different. if your speedo was correct before the transmission change, it sounds like this is your case.