A 951 in a X4(or a HX) hull can give it a little lean to the right. This can be overcome but I just wanted to mention that while your still mocking things up. It won't be so bad with the white pipe your starting with but the Coffman's will be lighter and not provide as much weight on the left. The battery will likely be relocated to the left since the exhaust will now be in the rear right so I wouldn't opt for a lightweight one.
I was wondering what kind of ignition you plan to go with. I'm gonna build up a 951 also(although not so over the top) and I've been piling up AM parts. I bought a world champion XPL hull for it even though I've never even ridden the parabolic yet. Anyway I do know the MSD Enhanser will run the 951 if you use a 98XPL cup. I also have a Micro Touch 951 ignition and the MSD Total Loss but have not read up on how to use those yet. I assume you will need something like that that's adjustable.
Your power plant has the potential to push the X4 to 70 or even a little over. I've read where the stock tunnel can be a restriction at those speeds and possibly not supply enough water to that big Mag pump. The handling is key also. I've been fine in the early 60's but you will need to make sure your hull, sponsons and trim tabs are true at the speeds your gonna be able to reach.