1995 La Brisa with the Pro Boss 351w.

I get basics of how to install.

Motor (ECU, starter, ignition, fuel pump?) go to battery A.
Accessories (lights, stereo, bildge, blowers, etc) go to battery B.
ACR connects + of both batteries, allowing battery B to charge when battery A is above 13v.

Now, what is the easiest way of splitting the system, without rewiring the damn thing? Stock there is one heavy cable that goes to the starter solenoid and fuse block with 3 fuses on the back of the engine. Another small gauge wire goes to a relay. Is there an "easy" way to split the power on the boat, or do I have to do a bunch of rewiring?