Wiring the alternator output post direct to battery positive through 4 AWG copper cable will result in less than 1% voltage loss across an estimated 3-foot run with a maximum alternator output of 150 amps. Protect the new 4-ga charge cable with a 6" long, 8-gauge fusible link at its connection to the battery. The #2 regulator (sensing) circuit can then connect directly to the alternator output post through a short, 18-ga jumper with no ill effects on battery charging. There is no need to protect the regulator sensing lead with any type of fuse. With this upgrade/modification, eliminate circuits 2C/2G running between the alternator and the firewall junction block.
Other beneficial modifications might include new 2-gauge or larger, copper battery cables; upgrading ckt 2J to 6-gauge between the starter and junction block (protected by a 10-gauge link at the starter); upgrading ckt 2E to 10-gauge between the junction block through the bulkhead connector, to its splice under the dash (protected by a 14-gauge link at the junction block); and separating the 12-ga headlamp circuit 2D from that same under-dash splice to reroute it out through an unused cavity in the bulkhead connector to the firewall junction block through its own dedicated 16-gauge link.
But, whether you choose to upgrade the power bus into the cab (circuits 2J/2E) and separate out headlamp circuit 2D, depends on what you have visualized for future current load placed on the wiring.