Alternator amp rating.
Disregard the halfaѕѕed advice from the Chevyheads at MAD Electric; there are numerous options for doing the upgrade properly. See
here and
here and
here and
here. An ammeter update can be done very cleanly without appearing hacked; how much it costs depends on whether you do the work yourself or have it done for you. Places like
Redline Gauge (lousy website; guess they're too busy doing actual work) or
Williamson's Instruments can do a stealth conversion of stock ammeter to voltmeter for "it must've come that way from the factory!" appearance as seen on
this '62 Plymouth (also custom never-available-from-factory oil pressure gauge in place of the clock, and engine temp gauge calibrated with numbers instead of just "C" and "H").