Your mention of the SBC383 is interesting as the SBC engines and the Chry R/RB engines have pretty much the same ignition timing needs. Both at 38 degrees BTDC optimum advance. Although Nick (Nick's Garage on YouTube) usually ends up with 36 degrees BTDC on the B/RB engines he dynos. So backing the timing back to 21 degrees BTDC would certainly mean a large power loss, as evidenced by the dyno numbers. Although there might be some more left past 31 degrees BTDC total.