Big 3, ’89 saab 900

My car is a '89 saab 900. I was looking at the wiring today. The battery positive splits into 2. 1 to a distro block for the accessories. The other then goes to the starter solenoid. The alternator piggy backs off the starter solenoid.
batt-----starter-----alternator

All the wiring is small as its only a 80A alt.

I will be upgradeing to a 150A alt. Should I replace all the wiring, or just run a length from the alternator, directly to the battery? (and leave all factory wiring in place)

The negative wires seem straight forward.

I just ran mine directly from the alternator to the battery and left the factory wiring like it was. Fused, of course. I've had no problems with mine. 

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'85 Toy

You placed a fuse on the battery end of the new wire?

So the alternator now has two wires, one to the battery and 1 to the starter solenoid?

2 questions, 2 yes's. You want to fuse any wire leaving your battery, close as possible to the battery.

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'85 Toy

What size fuse would you recommend?

Look under "car audio" on the left side of this page. Click on "power and ground" and you can figure it depending on lenght of wire, current, etc.  

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'85 Toy