My 10.5:1 c.r. 351-C runs best with 28 degrees total advance & 15 degrees initial on a fully warmed up engine. After setting these, I reattach the vac advance and adjust that curve, too. I get over 20 mpg@ a steady 90-100mph w/no pinging.
Most modified Clevelands with closed-chamber heads and some sort of aftermarket cam give best results with 30 degrees or less total ignition advance, due to the extremely efficient combustion chamber. High advance is a SBC setting for their less efficient heads, or for drag racing, not continuous street/highway use with a Cleveland. You'll very likely find this on the dyno during final tune.