Is the head for the 2.6 liter and the 3 liter the same. I know the bore is larger on the 3 liter but it has the same stroke as the 2.6?

no. when i was at the dealer we had two cars in for cylinder head work, one was a 2.6 and the other was a 3.0, somewhere at the machine shop they were switched. so the guy doing the 3.0 installed the head for the 2.6 and the guy with the 2.6 installed the 3.0. i don't remember exactly how it was, but i do remember that one had awsome power and the other wouldn't get out of it's own way. the combustion chambers on one was smaller, this increasing the compression ratio, you would have to compare the combustion chamber sizes or have then cc'ed to figure it out. on the better performing unit i don't recall if there were any issues like pinging etc.. they figured the problem pretty fast and switched them back.