Been there done that! Mine was a gas gauge in a 67 Mustang that I replaced,
like you, with a another gauge from a spare cluster. Put every thing back
together only to discover that now none of the gauges worked. Found that I
had blew the 5 volt voltage regulator. Replace the regulator, with the one
on the spare cluster, same thing, blew that too. Like a dummy I went out
bought a new regulator same story! I had now gone through three voltage
regulators and still had dead cluster.
What I eventfully discovered was that my replacement gauge was shorted to
ground through the mounting screws. I remounted the gauge and checked with
an ohm meter and that fixed the problem. Also, I replaced the mechanical
regulator with a solid state one from Radio Shack part # LM7805. I think the
mechanical regulator was around 30 dollars and the solid state one was like
a $1.20.

Your circumstances may be different as I believe the 70 Mustang cluster is
mounted on a printed circuited board where as the 67 was hard wired. But for
what it's worth that's what happen to me. Good tech articles on the subject:

http://www.mustangandfords.com/techarticles/75259/index.html
http://home.att.net/~marksmoore/Graphics/Bradster/ModifyCVU.gif