I am not sure what has been published but, I have parted out at least 15 66 cars and a few 65 and 64 cars both V8 and slant six over the last 6 years. I have never seen a factory installed drop center link in a 65 or 66 slant six car. That doesn't mean it didn't happen, but by no means are the drop center link bars in every 66 car. I own both a 66 and a 65 Valiant slant six cars, both of them have the straighter slant six center link. I always pull off the drop center link bars from the cars I part out, they sell for about $100 on eBay.

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1966 Signet
Turbo Slant Six

Even though the diameter is greater for the V8 center link, the link might might not be any stronger (carry any greater load in compression) than the slant centerlink.

Consider this...........when a left hand turn is made the pitman arm pushes on the center link to rotate the rear of the right tire. Consequently the center link is in compression. If the link is fairly straight it will not buckle as much as an offset link. An offset link causes eccentric load on the link and induces buckling at a lower load than a straighter link.

Said another way....Mopar increased the diameter because the offset V8 centerlink tended to buckle easier than a somewhat straighter slant link.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/