...The 340/360 heads have bigger valves and will work on the 318/273, but will require a notch in the bore for valve clearance for the 273 because of it's smaller bore...
This is not entirely correct. For 2.02 intake valves on a standard bore 273 you should notch the bores .070 max. For 1.88 intake valves lets do some math. 2.02 - 1.88 = .14. since we are talking about a radial distance, not a diameter .14 / 2 = .07. Or when using 1.88 intake valves on a 273, no notch required. Any over bore increases the clearance. That said, things are getting close so you should check before you slam everything together. 318's do not require a notch, ever.
As for over boring blocks, I hunted and searched for standard bore blocks and paid a premium for them. I always wanted the cylinder walls to be as stiff and strong as possible. You can get more power from using torque plates and good rings than boring a block .060. I only bore the minimum to get straight and round cylinders after everything is bolted together.