I believe that fig. 5-108 in the manual shows that the shaft is equal in length on either side of the lower arm, It looks like tool J-8808 locates the shaft centered in the arm, but I’m by no means an expert in front suspension work of the 61 to 63s.
As far as getting more positive caster, yes, moving the top control arm straight back by changing how the shaft is centered in the arm, should result in tilting the top of the steering knuckle back, thus increasing the caster. Looking at the service manual, factory caster specs at curb height are -3/4 to -1 3/4 changing to +1/8 to +1 1/8 degrees at full load (4 passengers). Changing that to a positive 3 or 4 degrees at curb height would help a lot in road feel and wandering.