Evan, the fusbile link is sized for the existing wires. You should not simply fit a larger fusible link unless you change all the wiring. In other words, the 20 gauge Fusibile Link protects a 16 guage wiring system. If you fit a larger fusible link and then get a short, the link won't blow before the harness. Not good.

Note that the original Datsun 1200 fusible link is green, and is attached to the Battery cable, not part of the wiring harness.




Both green wires are the same. Top: New, Bottom: old and faded

If you want to switch to Circuit Breaker that is fine. Do it as close to the battery as feasible and make it an 85A unit, as the stock wiring harness and fuse box is 85A total. If you want to add more capacity then add a 2nd circuit break to protect your new additional wiring. The stock wiring presumably will not handle more than 85A.