It all depends on what you want out of your conversion.

But you don't need to replace your harness. 

-the 6-volt harness is of a heavier gauge than the 12 volt.

If your 6v starter and flywheel haven't been replaced you don't need to either.   I had a 6v starter on my 12v system and it worked great.

What you need to complete your 12v conversion for your car to operate correctly is...

12 v Generator/alternator, coil, and carb (with choke).

As for accessories you can put resistors on all the 6v bits or change the bulbs to 12v.

You will need a 12v wiper motor or you can use a 67 wiper motor and switch.

If you "must" re-wire your 58...look up Wiring Works Kit.

If i were you i would work with the wires you have now.  If they are beyond repair or just need a new harness you can get a 74 harness.

For wiring the Alternator with your old harness look here: 
http://www.nls.net/mp/volks/schem/gen_alt.gif

Yeah if going new.

I would go with a 74 harness.

Although a 58 would be almost the same. 

Many different ways to do the conversion.

we need to make it look something like this...

I can snag a bunch of weather pack connectors if you guys want to get all fancy like.