I just bought my first 58 Plymouth Belvedere. I'm assuming it is 12 volt negative ground but just want to be sure before I put a battery in the old girl.

Should be 12V. Plymouth went from 6V/ positive ground to 12V/ negative ground in 1956.

It ought to be, 55 was the last year for 6v.

FSM and parts manual available here

http://www.mymopar.com/index.php?pid=31

Thanks for the help

55 was last year for 6 volt.56 was 12 and neg ground.We have had both One 47 I had we ran a 9 volt battery.Rocky

Our '57s were 12V Neg ground, like a proper automobile dammit

Most European 12 volt cars back then maintained the negative side as the hot side, not negative ground like American cars switch to when they converted to 12 volt. GM converted Buick, Cadillac and Oldsmobiles (SP?)to 12V in 1953,not sure about Pontiac and they converted Cheverolet to 12V in 1955.
Mopar and Ford switch all their cars to 12V in 1956.
All the current in battery flows from the negative side back to the positive side, they only flow from positive to negative internally Hence the importance of proper and good grounding
Both ways work

positive ground encourages galvanic corrosion and shorter plug life.

While they both work, negative ground works better.