The output from the mechanical speedo cluster to the ecu will work fine. It matches the output from the electronic gearbox speedo drive that normally goes straight to the ecu and electronic speedometer in a Rev3 car.
 
I have a Gen 3 3sgte with factory ecu running in an AW11 with a mechanical speedo. No issues at all.

Mrskylighter
The output from the mechanical speedo cluster to the ecu will work fine. It matches the output from the electronic gearbox speedo drive that normally goes straight to the ecu and electronic speedometer in a Rev3 car.
 
I have a Gen 3 3sgte with factory ecu running in an AW11 with a mechanical speedo. No issues at all.

But my mechanical speedo cluster won't work as my new gearbox has an electronic sender as it's the rev 3+ lsd one. So I need an electronic dash or a mechanical sender to install in the box.

Yep you'll need to make that decision. Mechanical speedo sender probably the easiest option than re-wiring some of the cluster plugs.