I just measured the port diameters on a Harper manifold and a 261 850 head. The ID of the ports in the Harper manifold is about 1.560 +/- 0.010", with no counterbore for the alignment rings. The OD of the alignment rings, installed in the counterbores in the head, is 1.500", with an ID of 1.39" +/- 0.01". The ID of the stock 261 manifold mirrors that of the head,
Thus, it seems that the Harper on ebay would fit right up to a late 235 or 261.
