The reason why they want you to go a closed chamber is that is the ONLY way to get good compression on a 402 or 396. The smaller the bore, the harder it is to get good compression. The open chamber at 118cc will give you less than 8.5 to 1. It would need to be shaved A LOT. You have the same pistons as I do in my 402. I have a 96.4cc chamber, large oval port head. I have about 10.5-1 compression.

By the way- having the closed chamber piston means you can run either type- open or closed chamber head. If you had open chamber type pistons, you would not be able to use closed chamber heads. This is due to the shape of the dome.