7:1 head is 3.700 in thick, chamber volume is 91 cc.
8:1 head is 3.600 in. thick, chamber volume is 75.2 cc.
7:1 pushrod length 7.300 in (I am yet to find a part number for this, according to my books, all 2.25 petrol use part no. 546798, S11+11A+111)
8:1 pushrod length 7.175 in
Milling .100 in. makes a 7:1 head 8:1. Almost exactly.
If you modify a 7:1 to 8:1, use 8:1 pushrods. Otherwise the rocker geometry is off, and the possibility of fouling the rocker exists. By rocker geometry, I mean the angle that the rocker arm contacts the tip of the valve stem. If the longer pushrods are used in place of the shorter (correct) ones, the rocker arm depresses the valve with it's very tip, concentrating wear and forces at a place where the rocker was not designed to be operated. This can cause premature wear of the valve stem, unusual wear of the rocker arm, and failure (breakage) in extreme cases.
By Hank Rutherford, Steve Denis, Peter Ogilvie