The camber adjustment on your 66 is done through adjustment of the eccentric on the inboard part of the lower control arms. It is not done on the upper control arms.

The problem is you are probably looking to dial in some positive caster, and are limited by tire clearance due to the 66 Chevrolet style wheel opening. Stock alignment calls for basically zero caster. 3-4 is the new number on modern tires. Going positive can really transform the straight line feel of the 65-70.

Believe it or not, I've actually seen the use of shims between the upper control arm shaft and frame at the rear bolt position on a 66 Biscayne. Done exactly for the clearance issue and a lack of other options. Unconventional for sure, it moves the upper ball joint back a wee bit, and allows just a little more trade off on pulling the lower arm forward into positive territory, while staying away from the fender. Some aftermarket upper arms actually come with the upper ball joint setback a little more than stock.

Try a 60's series tire?

Or just use the stock alignment specs and constantly saw the wheel back and forth, a-la 60's style.

 

You can really see the issue the 66 Chev guys encounter with this image,

No idea on the size. Snagged from the net.

I guess if you have a spare upper arm with shaft you could mock it up to see how much you gain from shimming. No idea of how much it would move.

LT1 can't pull much positive caster because he's limited by the fender corner getting too close to the tire. By style the 66 Chevrolet full size have the issue with the curved back fender corner. Stock settings were fine with narrow bias plies and wheels, but pull the lower arm forward (positive caster) with a wider and taller tire and you have a problem.

I've heard the issue discussed here and there over the years on the Chevy forums in the pursuit of more positive. Shimming the upper is just a potential workaround on the issue.

I made up a little triangle control arm mock up. And it appears any shimming at the rear cross shaft mount will actually give you a rising ratio at the ball joint as it moves back. Which will probably translate back somewhat less down at the spindle. So in theory 1/4" of shim may be close to as if you had moved the lower arm forward the same amount.. give or take. Or look at it as if you had been able to move the strut rods nuts down the shaft a similar distance. 

So on the alignment rack set the rods to the point where the tire has adiquite clearance. Set camber and toe close. Take the caster reading. If too low (below 2.5+), loosen all the upper arm shaft nuts and drive a equal thickness shim in between the frame and shaft at both rear mounts. Tighten it back up, reset camber and toe, and see if your caster numbers improve. Go from there.

Sorry for the long rant.