The problem with 6X heads is chamber volume and MOST importantly large valves. What is needed is a 1.96 / 1.66 and even better is the 1.88 / 1.60. The problem with 1.88 / 160 is the bathtub/valve shrouding chamber design, so the 1.96 /1.66 open chamber head is the best bet.
Large valves are fine in a performance application, but if you are going for mileage you need port velocity at low engine speed and the small valve small port diameter does the job better. That is also why I recommended the performer small port intake manifold. That is also the reason Cadillac engineers designed the 472" and 500" engine with small valves to be able to keep the torque and have decent mileage for a 5,000lb. + car.
D.
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