Here's a picture of a 287 manifold. Doesn't look flat to me. The differences between the '70 and '71 engines are the heads (290 heads flow well and have a good combustion chamber design, 820 heads flow okay but the combustion chamber is sucky; not sure about the other '71 heads), camshaft, and 1.7 points of blueprinted compression (11.3 for the '70 and 9.6 for the '71). See
www.classracerinfo.com for the actual calculated compression ratios.
