Weight and balance datums

Does anyone have the numbers for common datums for the major components? I know the station 0 line is 70" in front of the the forward wing root, but what about prop, engine, and firewall? I'm trying to plan a build and know it's going to be a fat pig. I wanna play around with some engine and prop combos to see the best setup but I don't know where they're located. For easy of calculations, I'm just going to assume center of physical dimensions for cg of components, so front datum or center datum will be fine. Thanks.

Use a plum-bob to mark the 70" with the aircraft level, then measure back from there to your firewall and other locations. You'll need to do this for the landing gear too.