Wouldn't it make that noise all of the time? I only notice the spark knock noise under heavy load going through the gears. Not WOT but a good pedal pressed down.
I know it sounds strange but mine only made noise on accelleration too. After I installed shorter pushrods the noise went away. I just read your post below where yours are clearing the pushrods. Do they have excessive side clearance that is allowing them to slap back and forth against the hold downs? Mine seemed to have alot of side clearance I noticed.
If you don't find an obvious problem at the rockers to make sure it isn't spark knocking maybe you should check the timing to make sure something didn't slip there. Maybe try backing it off 3~4 degrees degrees just to see what happens. That'll rule out if it is actually spark knocking or not.