Roush S-351 Cluster Question

I am finishing up the Faleen and when I installed the S-351 Cluster in it I have a small issue. The Speedometer section reads correctly which I knew it would since I took it out of an S-351. The Issue is the Tach. It is moving smooth at all. Free Rev it sordof skips and reads off. When you turn the ignition off it doesn't drop all the way back down it hangs at 500-600RPMS.

I have put a stock GT cluster in the car and it works just fine. I was wondering if I took the GT Tach out and put it in the Saleen Cluster and used the Saleen Need if everything would be good to go? I figure it would be but wanted to double check. Also just wanted to see if anyone else has had this issue.

Thanks for any replies. I will get pictures up of the car as soon as I get it finished up.

Basics about it.
408W
TW heads
Custom Cam
S-351 Upper Intake
T-Trim SuperCharger
my favorite - Coil on Plug iginition running 99 OBD 2 EEC.