True true words from JR! It is more than worth your while to swap that gauge in. I have run ammeters on stuff with generators, regulator/rectifiers and they are worrysome (to me).
If you search posts in here, you should find most of the info you seek. You can daisychain the connections at the gauges themselves, so you only have one 12 volt key-on feed, one ground feed, one illumination feed, etc.
For illumination you can hit the dimmer wire at the back of the headlight switch or fuse 13. If using the latter, use the dead side of the fuse and fuse your new wire with the smallest fuse you can find (so if your wiring is bad, you hopefully dont take out your stock cluster illumination - so you can see your stock instruments till you get home).
If you cant find a large hole in the firewall for the sender bulbs (autos should have an extra hole where clutch linkage would go), then a step drill bit (unibit) might help with the hole drilling.
Good luck.