Not really Nick. Not the place here really for a detailed explanation, but alternators don't work like that. Say your truck with everything running pulls 40 amps. I mean headlights, batteries, heated steering wheel.... You have a 70 amp alternator. So all is good. It's within capacity. But you then decide to fit a million amp alternator. With everything running you're still pulling - yep, 40 amps. Now of course if your vehicle demands then increase because you fit a sauna, massive stereo, floodlights etc and you're now pulling 120 amps your little alternator is going to be maxed out and your headlights will be a bit dim I'm afraid but you can still only supply 70 amps. So you fit a 150 amp alternator. Hurrah, you're back in the green with a bit to spare.
Now will you have to change fuses and such? Well not really as each individual item such as headlights, heater, stereo will still only demand the same current. It's only the new things that you fit that will need checking to make sure that the cables are right.
Unless you do a calculation of how many amps you actually are drawing and it's very close to the 70 at full whack, you won't see any benefit from a bigger alternator.
That's a simplified version of course and not one from an electrical engineer. But safe to say unless you fit something that sucks huge current you don't need a bigger alternator. There's only so much you can cram into a battery in any case. Fitting a billion amp alternator won't charge your batteries in any quicker time than now really.