boring my 318

The 340 crank is the same stroke as a 318 crank.
The '73 crank is a cast unit and extrenally balanced. This is a 1 year only item. It is possible that a late '72 crank could be this way. The machinest will know when he see's it.
Otherwise, a 340 crank is fordged and early 318 cranks are as well. I'm not sure about what year they are cast. (I forget and I do not have a book infront of me. )

Overboreing should be keep to a min. While it doesn't really contribute to a ratio increase, it will in a most modest manor. It is more of the piston height that changes the ratio. Just ask for a 9.0-1 piston with the head you have or the amount of cc's the head has.

Max bore is actually a changing amount determined by the thickness of the cylinder wall which can be check by a (Good) shop with a machine. It's called sonic checking. You don't want to max out the bore. Thin walls can lose power by flexing under stress. A thicker wall is prefured. The amount of cubic inchs gained by an overbore and power gained is nothing and if gone to far, will offset and modest gains achieved.