Anything special about a 91 318?
Look for a large I cast on the side, for Industrial. I have one and the cylinder walls are over 5/16 thick on the major thrust axis for the first 1-1/2 inch from the deck and then taper to over a 1/4 inch thick for the remainder of the cylinder. It would easily take a stock 340 piston and still have at least .185 on the major thrust axis.