[Ken Bergen]

It will fit however there are some wiring differences.

Your 84 had the pulse generators in the rear rather than the front and the Aspencade has six wires coming from the gear switch rather than the three on your Interstate.

All this can be overcome. Also the 85 and later GL1200's are lower geared than the 84.

 

[Ken Bergen]

were is the switch gear located

The gear indicator switch is inside the front transmission cover next to the water pump.

There's likely no reason to change it, just cut the six wires and connect the three you require to your old plug. The colours of those required wires are the same on the Aspencade and the Interstate.

 

[Ken Bergen]

How loud is the tapping. i'm told the 1200 have noisy valve trains
Wilf

And who's the liar that told you that?
If the automatic adjusters are shimmed properly they're as quiet as a mouse.

 

[Ken Bergen]

My 86 interstate has a 6 wire plug for the transmission plug. I put an aspy motor in it and it had the No wireing needed. The pulse coils are different. But I'm not sure about the gear ratio. I thought all 1200's were the same.

As far as I know all Interstates had three wire gear indicators, N-4-OD. The wiring diagrams in my Honda manual say so.
And yes the 84 was a one of gearing. That was changed in 85-87 supposedly for better fuel economy but are lower geared than the 84.
Takes the edge off I want a sixth gear doesn't it.:?