I agree with Tony, if it ain't broke don't fix it. Depending on where your voltmeter is wired in you are more than likely seeing the effects of volts drop along cables and across less than perfect wiring connections. If that is the case then fitting a higher output capability alternator will do nothing to cure it.
Not sure what alternator you have fitted currently (pun intended
) but if it is the standard 17ACR which I think outputs around 36amps then
full beam headlights 20 amps (maximum of 4 x 60W = 240W or approx 20 amps)
sidelights 3 amps (6 x 5W = 30W or approx 3 amps)
Ignition 4 amps
O/Drive Solenoid 1 amp (maybe a little more)
Total 28 amps
Still leaves 8 amps headroom. Not a lot, but unless you do loads of driving in the dark and wet (rain = wipers & blower) and/or also do loads of very short journeys with lots of stops and starts, then that should be plenty even allowing for recharging the battery after starting the car. (Assume starter takes 400A for 10 seconds = 1.1A/H from the battery, so battery needs about 1.5A charging for an hour, in reality it gets 10's of amps for the first few seconds after the car has started so most is recovered fairly quickly and the rest trickles in on the journey.)
It is only necessary to fit a larger alternator when you start fitting power hungry accessories. A common one is an electric fan - these can easily "pull" in excess of 20 amps - but they don't usually run all the time, so again depending on how you use the care an standard alternator can cope. My Landy 110 2.5 Turbo Diesel has its original 45 amp alternator and it is fitted with twin kenlowes and I tow a large trailer at night - never had an issue with a flat battery - and it kills them on starting needing glowplugs for 15 seconds before starting at 80A and massive starting current.
One last point when fitting higher output alternators to old cars - the original design and wiring, terminals and connections was for a 36 amps alternator. Simply swapping for one with nearly twice the output risks putting excess current down "tired" wiring that wasn't designed to cope when new let alone when it is a bit tired, result - at best warmer wiring! So if you do swap for a higher output alternator you should carry out a thorough check of the wiring.