As stated above there are many variables.
However, I did test this on a similar powered 440 on our "loaded" chassis dyno 2 weeks ago and discovered the following:
1. The RWHP difference between 34 degrees and 39 degrees was 35hp (+35rwhp at 39).
2. The RWHP with every degree added from 38 to 40 was only 1 RWHP per degree.
3. Retarding timing at 6580rpm from 39 to 36 increased rwhp by 3hp.

There were many other tests but I won't bore you. This was using Citgo 110 race fuel in a 12.89:1 446 w/MSD Dig7, full exhaust, no air cleaner, blah , blah....