Right - it's a piece of cake
RE wires for the rev counter - 1 is a feed from the ignition,1 is earth,and the third wire is a feed from the negative side of the coil (the pulse that makes it work!)
As for the rest of the wiring,if you've got the connector that fits the back of the 7 dial clocks (10...ish pins?) ,the colours of the wires on the existing "clapometer/2 dial dash) marry up ie green and white goes to green n white etc.
If you've not got the plug,either use the Haynes manual schematic wiring diagrams
,scream and rant,or PM me and I'll take the colours off my spare 7 dial plug and try and help ya out
Dave C