Have you tried doing it the other way around? Smaller 680 jug on the longer stroke 603. Could have a lot of torque with the extra stroke. Rod the same length?
I did try that a while ago, it ran for a couple minutes too lol. I need to make a better spacer and figuring out the port timing was a pain in the azz, I tried several 52mm pistons, even made a couple pop ups to try.