Sorry to be late to the party. This forum area would not load for a while today. It's been a long time since I messed with either model so forgive me if I'm not completely remembering the details.
The way the boards attach are quite similar, with one lock screw and two latches, but images on some repair sites suggest that the ribbon cable connector on the Titanium is almost twice as wide as that for the Pismo, so there may be many more wires.
I cannot find the dimensions of the two boards. Obviously, fit is a prime consideration.
Searching the web for "mac mods" will get you to sites that specialize in custom configs and part swapping. They may be more useful.