I have a 944 tach that I re-worked for a Pre-A dash, the size is the same as the hole in the Speedster dash. Replaced the plastic cone with flat glass, Easy Peasy.
Here's a pic or two:
http://img.photobucket.com/albums/v136/TeamEvil/dash5.jpg
http://img.photobucket.com/albums/v136/TeamEvil/dash4.jpg
http://img.photobucket.com/albums/v136/TeamEvil/dash9.jpg
The tach wires very easily. +12 is the plus wire (12 volt key on) coming in from the harness. 1 connects to the 1 terminal on the coil. - goes to ground.
On the combination gauges you usually run a 12 volt key on lead to the + and a wire from the sender to the G (or S) terminal. This would go for all three of the gauges in the combination. The gauge body has it's own universal ground spade I believe. The usually do.
The speedo takes a late model Beetle cable, a Ghia cable offers a bit more length if you need it.
And, of course, there are wires and plug-in bulbs for the hi/lo beams, directional indicators, panel lights, and various idiot warning lights.
Hope that this gets you started, someone might wanna jump and confirm or deny, I writing from memory. I can double check the dash wiring if necessary, but I THINK that I've got it right.
Luck,
FP