I disagree (as usual).

RAIII and HO share the same RPO 348, the same engine codes, the same block, crank, heads (48 and 62 depending on A/C or not though some sources also list 16), same intake, same transmissions, same cam (067 auto, 068 manual), long branch exhaust manifolds, distributor.

Documented differences are 4 blade fan on RA, 5 blade (no A/C) or 7 blade (with A/C) on 400 and HO.

The most distinct difference is the carburetor: 400 and 400 H.O. used 7029263 for manual, 7029268 for auto. With T-42 Ram Air option, 7028273 was used for manual, 7028270 for auto. The RA IV used 7029273 for manual, 7028270 for auto.

And, of course, the T-42 fresh air package (RPO 611), which is associated with the change in carburetors, turning a 400 H.O. into a Ram Air III.

Reference: 1969 AMA Specifications