With the same heads once you go over 4" stroke you're rate of gain per cube increase will slow down, above the torque peak the RATE of acceleration slows as the VE drops, in your case (same heads and compression)the longer stroke isn't getting the proportional rate of increase in airflow to offset the increase in frictional losses.

Long stroke small blocks like Andy's example are built and tuned to achieve the highest average torque, and a flat broad torque curve without much variance (achieving at least 85% of peak torque over a ~2800-3400RPM band...the right intake manifold is important here too)...peak HP is not really the primary target. If Andy wanted more hp at that approx size he would have bored that 340 Resto (siamese) block to it's max ~4.22" IIRC) and shortened the stroke to something like the 3.79" , plug that in and you should see more HP per cube but a little less peak torque...and at a somewhat higher RPM...if you really examine it you'll see the frictional losses are lower....because the piston speed is lower at any given RPM. This is most apparent above the peak torque RPM.