Xavier Any chance of a first name? Anyway 2 thing are used for gauging charging cable, amps and length of wire from point "a" to point "b". Alot of rod rebuilder use this for wiring cars with extra power needs. for a 65amp alt and an 8' run to main connection, #6awg is safe, can run #8awg but depends on load. 80 amp and up would be #4awg. Again you can get away with a #6awg. A mega fuse is a slow blow fuse to protect the whole car from a grounding of the charging cable, over charging ALT, or short to ground. I have seen these save wiring harness. 125 would be perfect for a 100 amp setup. 100amp would be ok for a 75 amp ALT. One other quest is how thw wiring is setup in the car. If all the power goes through the amp meter first like ERA, this is why you want heavier cable to handle the load. By running a output cable directly to the pos of the battery the amp gauge will not work and you have no idea about what's going on. This is why a volt meter is better in some cases. Again it's not period correct but neither is my car.
Battery's I am not a fan of the overpriced Optima batterys. The thing with them is mounting them in any position. As far as maintainance, the one I had, use to have a trickle charger on it when the jeep was not in use. Red top 2 1/2 years and bad battery. I run Delco or interstate batterys now. I still have a small trickle charger on the car when not in use. I can buy 2 delcos for the price of a Optima and almost the same CCA rating. I have never tryed a gel cell battery. Any way, this info is to protect your car. You can run less and alot do but,------.
Rick L.