You're on target with +3-4-5 positive caster. See what you can actually get (alignment shop).... +5 would be great (with about 0.75-1.0 negative camber, and ~1/16" toe-in). The factory spec was pathetic, as well as intended for cars without such a thing as "adjustable UCAs" (they didn't exist back then.... also, the factories wanted drivers to slow down and not drive quickly through turns, etc.. most drivers couldn't handle such style of driving, nor was there the available higher +caster designed into those factory UCAs.
If you have power steering then go for the high-side (+5); the extra effort will barely be noticeable... don't be afraid of ~ +5 caster. I know someone who has about +7 caster, manual fast-ratio steering, used on a road course car (GT1 '67 BCuda)... not bad at all while driving.