GPSS options with 400A

[cessnadriver]

Hello everyone. Quick basic question. I have a '65 C337. When I purchased my plane, it came with a Nav400, IN A BOX. I was advised, from a hangar neighbor, that Autopilot Central is the place to have the 400 bench checked/repaired. My question is what instruments drives the Nav 400? The AI, HG or turn coordinator? The reason I'm asking is I'm sure that those instruments have been changed from the originals. So I'm looking at pricing replacement instruments? I don't want to spend the money to install a $30,000 S-tec in a plane that I paid $22,500 for! Actually after buying a overhauled Cont IO-360C and six rebuilt cylinders for the other engine I'm in for over $40,000. But still If I wanted a $70,000 aircraft I'd buy a newer C337, that's better equipped, in that price range.
Thanks in advance!
Regards,
BILLS