Johnson 70 1988 Tacho, trim gauge and voltmeter

Just fitting a 1988 70 Johnson J70TLCCA to my boat and my boat has a tacho, Voltmeter and trim gauge. Any idea on wire colours for this or is there a loom available to plug into the Control box?
Secondly, the measurement from the top of the Transom to the bottom of the boat is approx 201/2 inches and the engine from the top of the transom bracket to the bottom of the cavitation plate is approx 181/2 inches. Do I need to cut down the Transom or see how it goes first?

If your control box is so equipped, there is a loom for the tachometer to simply plug into it. If not........

There will be a "gray" wire protruding out of the instrument wiring harness (harness from engine to dash). This gray wire is the sensor lead for the tachometer.

Tachometer wire colors if tach has leads protruding from it, are usually: Gray (sensor), Blue (light bulb), Black (ground), and Red (voltage). If no wires, tach terminals will be marked: S (sensor), L (light bulb), G (ground), and usually V,+, or B (voltage).

Sensor (S - Gray) - To Gray Lead
Light (L - Blue) - To Light Switch (Usually bow light terminal)
Ground (G - Black) - To Absolute Ground (Not to either "M" terminal of ignition switch)
Voltage (+ - Red) - To "A" Terminal Of Ignition Switch)

Top of transom bracket to bottom of cavitation plate? The measurement should be from the bottom of the transom bracket where it seats upon the transom (perhaps this is where you took the measurement?). At any rate, the measurement should be from that location to the top of the cavitation plate.

The "L" in the model number of J70TLCCA indicates that your engine is indeed a long shaft engine that should work well with the transom measurement (20-1/2").... try it before getting your saw out.