Rpm 340 Cyl heads on a 318 !

By notching the bore- I assume you mean "fly cutting the pistons?" or actually notching the edge of the cylinder wall to accommodate large dia valves?

If you notch the pistons get them notched for the valves you are running not oversize. Different diameter valve heads will strike the pistons at diffrent spots and depths.
Easy thing to do is spin cut a valve shaft to a take the valve head off and then bring it to a point.
Assemble the bare heads on the block and with the piston a TDC use the new valve to make a prick punch in the top of the new piston.
This will let the machine shop know the true center of the valve so they can fly cut the radius.
There are plenty of kits out there with all this work done for you. Just running a diffrent cam profile can change stuff.
Good luck,
Joe