So I just got my new auto meter marine gauges for my boat, and while I am at it I am going to re-do the original 1976 wiring harness.  I have a couple questions though, first, how should I go about rewiring the original ammeter for the new volt meter?  If I am going to be redoing the harness, can't I just delete that wiring altogether?  Second, would the new gauges be compatible with all of my old senders? My old gauges are basically junk, and the only gauge that I hooked up was the fuel gauge, and my old one read 1/4 tank while the auto meter is barely above empty.  Is there a way to adjust them?  I am fairly certain that there is more gas than that in the tank.  I appreciate all the help I can get on this!  Thanks!!

sending units and gauges use ohms to send levels, make sure your senders match your gauges.  water temp and oil pressure senders are very cheap and heck... why not make sure they're working 100% and accurate?   most water temp and oil pressure senders come in "ranges" so 80-180F or 0 to 60psi... fuel level senders are the same way ohm wise, for example:

http://www.cpperformance.com/p-211-2-wire-fuel-level-sender.aspx

This design makes installation and calibration for use with 33-240 ohm gauges.

if you look at jegs/summit they'll have the gauge and its ohm rating next to it.  you'll see some are 0-90, 0-280, etc. 

as far as swapping ampmeter to voltage i just wired my voltage gauge lead off of my ignition switch's ON, whereas ampmeter wiring is such that it goes battery - ignition switch - ampmeter - everything else.

dont forget to use fuses. make sure your grounds are clean and tight.  if you think a gauge might not be working you can ground the sender wire and the gauge should be a full sweep. 

Awesome!  Thanks for the replies everyone!  GT, are you referring to the harnesses that they have listed that all begin with the part number MAR6117?  If so, that is a damn good deal, and I was expecting to pay around that just for the supplies to build my own.  That may be a better route for me to go.  As far as the senders, I will look in to replacing them, so everything from the sender to the gauges is new, and accurate.  That may be my best bet.  I will probably pull the fuel sender and see what it looks like, and see if I can play with the float on it and make the gauge respond to it.  Thanks for the help again!