Xavier Allen we all have been in that boat of building or buying parts that will not work as they are told about. I have better than 30 years of this and still have gotten caught with issues of parts not being up to what I was told. My car is over build for a reason. I have some double systems as backup incase of a primary failure like 2 fuel pumps. I use one most of the time but have switch over to give the other one a work out too. Most cars had fusable links in the old days to protect wiring. Today they have mega fuses in the 100 to 200amp range. 3,000 rpms on most car is where the charging system is measured for max charging. No alt will put out 100% all the time unless either a battery cell issue or too small of a supply wire to the car. Startup you see high readings because the Alt is recharging the battery plus suppling power to the whole car. This is some of the reason batterys are getting smaller and starters are using less ampage to start a car. 175 to 225 on the old days. gear reduction was started and everything has changed, for the better. Saving weight. Rerouting wiring is a good idea, make sure you have them not rubbing against any parts of the car and have clamps or clips about every 12" to keep wires from moving, rubbing, or vibrating. Vibrations internally break the wire and you don't see it. The resistance goes up and so does the power load on the Alt. I will try and give the best info I have or direct you to someone more knowledgable in that subject. There are some great guys on this forum that have alot of years of building cars and knowing what works and what is questionable. The best thing I can tell you is do your homework and ask alot of questions any time you can to other builder and manufactures.
They are like Doctors, I always get a second opinion.
You are going in the right direction. Good luck. 75 amp will be OK without all the power ideams on at idle. It also depends on how hot it is out too. Rick L.