Voltmeter: I used Teleflex when I redid my C36 panel in 2001, but they didn't last. I'm not sure it makes a difference.
Starter to alternator: I would shoot for a less than 1% drop at maximum alternator output. I would never use anything less than 4 AWG. You should also check what you have from the starter lug to the batteries - length and gauge.
Stud size is probably 5/16", could be 3/8", don't bet on what I say! It's been a while.
Do you have an internally regulated alternator, or do you have an external regulator installed?