Yes, the alternator will only produce what is needed. But if you need to upgrade to 75A, then you must have a need for that many amps. If it was only going to produce 50A, then that's all you need and what would be the point of the "upgrade"?
So, going on the assumption that you need 70-75A, you might need to upgrade the wiring. You'll need to look at what you have already, compare it to an ampacity chart, and go from there.
https://www.cerrowire.com/ampacity-charts
If you need more wire, you can either remove the present wire and add the proper size, or add a second wire. A Painless harness has two wires. You connect the second one if you're using a high amp alternator.