When I put the 1 wire in mine - I used a piece of fine wire #6 welding cable (with soldered copper lugs of the proper size) and ran from the post on the alternator, clear around to the battery post of the starter solenoid. Voltage drop is virtually zero - the measured volts at the alternator output stud is just about exactly the same as at the positive battery post.
You will need to figure out a lead to the horn relay - as the horns run off a power source that comes back from the old voltage regulator. The regulator has a yellow number 10 wire that goes to the battery post of the solenoid - it was the original main chassis feed. You might use it to run power back to the horns.
When I put the GM one wire alternnator in - the directions from the manufacturer suggested that you arrange the pulley ratio to be 3:1. Meaning the crank pully to be 3 times the size as the alternator. I had to do a little shopping around to find a smaller alternator pulley as the T-bird driving pulley was fairly small relative to the usual GM size.
Steve Metzger Tucson, Arizona