You would be better off with replacing your gages with reproduction gages. temp. sending unit will have a longer capilary tube for better reach (does not run off of battery power). 6 or 12 volts don't matter for the Oil pressure gage either so you could use the original. Fuel gage can be bought as 12 volt, but looks the same as original. You can find the repo gages at any of the aftermarket sources e.g. Brothers, Jim Carters, Classic performance..
http://www.oldchevytrucks.biz/47_55/full.asp?page=164
For the amp gage, I reworked my original gage face with a volt meter. The only diff. is the needle, which is not very noticable. This is much safer that using a amp meter because with the amp meter, you need to run a 8# or 10# wire to the gage from the alt.. See link below for some pics of the volt meter mod.
http://67-72chevytrucks.com/vboard/s...d.php?t=318878