The #6 heads are just as good as the #5. Of course I say that because I own two sets😁. Kidding of course, they are so close you can't say one is better than the other. Let's see some pics, if a set is recently rebuilt with upgrades, $600 isn't out of line. I paid $800 shipped for a set of milled to 55cc, ported, will see soon how good the job is, 2"/1.625" valves, positive seals and double springs. I picked up a completely fresh rebuilt set, 62 to 64cc chambers, 2.05"/1.56" valves, upgraded springs with positive seals for $400. It really depends on the heads, work done and condition.