Fusible Link Dodge B200 1977.

Hello everybody.
I need to replace the battery terminals and cables on the above van I'm restoring in the UK. The terminals and cables aren't the problem - it's very straight forward.
As you are probably aware there is a fusible link off the positive terminal, which goes up through the firewall and behind the dash. The one currently on my van is, to say the least, shot!! It has been repaired several times and not very well - crimps and tape!!.
So, my question is how do I take apart the firewall multi plug to gain access to it's pin and secondly what size cable should I use to replace it? I'm a true novice at auto electrics, and I really am struggling to understand the use of the link and why it couldn't just have a proper fuse on it?
As always, any guidance will very much appreciated . . .
Best regards from the UK
Timbo
Author: Timbo