I have a cluster from a 1987 760 turbo. I want to put it into my 740 GLE non-turbo. I mainly want the voltmeter. My GLE cluster doesn't have one. Are these clusters interchangable? If not, please elaborate as to why not.

Thanks in advance

I switched volt and boost guages onto my existing cluster. the hardest part was removing the cluster. Wiring is cake. You might consider installing the boost guage as an indicator of fuel economy, engine, and air cleaner condition.

WHOA, BACK UP!!! LOL
You've gotten my attention. Please elaborate fully and/or point me to some info on doing this with a boost gauge.

I would LOVE to have a fuel economy indicator at this point to help diagnose my "sick-brick"!

You da man

Well, the empty spaces where the guages would sit in your cluster are black cover plates which can be popped out. you unscrew the cover on the back of the cluster where each new guage will be installed, and pull the plates out.
Slip your new guages into the holes, and fasten them down. The volt meter has 2 wires, red and black on the back. Note where they are connected on the old cluster, and connect them the same way. The boost guage (left side indicates engine vacuum, and it will point straighe up at full throttle) has NO wire, but you will have to route a small vacuum line into the engine compartment, and connect to manifold vacuum. The lighting just works.
there is a pretty goodbasic article on vacuum guage diagnosis here: http://www.fordf150.net/howto/diagnoseengine.php
If you remove the boost guage from your spare cluster, and examine its mounting, what you have to do should be pretty clear.