yes, you will need to notch the block. best to let a machine shop do the work if you don't have experience with this. BTW, P90 will give you pretty low compression on an L24. why not swap it out for a flat-top L28?
All you need is a mounted stone on a drill (barebones!). Do not notch beyond the fire-ring of the headgasket, and don't notch below the mark where the ring seats on it's upward travel and you will clearance fine and not ruin anything.
I filled the top of the piston with shaving cream to keep the grit out of the rings when I notched mine. I used a 2" dia stone held at 45 degrees to the bore and it cleared by a mile. The valves in question only nick the bore tops. Take a look at an L26 block to see what the Nissan solution was to the same predicament (only a 1/8" spotface-looking affair).