Re: '67 Alignment issues after '74 Disk brake swap
 

Caster IS NOT a tire wearing angle. PERIOD. The fact that he has no shims in the left side and "thin" shims in the right AND the camber is already .5° positive shows that NO MORE shims can be added to increase positive caster. Adding more shims to increase caster will only make the camber MORE positive. The lack of caster is the reason for the poor return to center. You will have to reduce camber before you can add any caster. If you are satisfied with positive camber (I wouldn't be) then you can mask the problem by doing the Caster mod.
Its time for pictures of the truck and suspension. Are your springs installed correctly? Are they the originals? What is your ride height?

Re: '67 Alignment issues after '74 Disk brake swap
 

Make sure the shop puts the shims in the right spot. When I had my truck lowered last year they put the shims between the control arm and the angled spacer- not the spacer and the frame... which made a big difference on the machine.

Re: '67 Alignment issues after '74 Disk brake swap
 

Well, I haven't taken a close look since I got the truck back, but I have to assume the few shims that are there are in the right place. Springs are not originals in theory, but are supposed to be oem replacements. (That's what the P.O. said at least). The main issue is the '74 spindle geometry with the '67 control arms. I know the balljoint-balljoint height on the '67 was 9" and the '74 is 8". I could live with a bit of camber if the driveability was better, or I may hunt for a crossmember.

How much camber do you think I'd have to get to get the caster in spec?