Need help with a loft or freeform feature

[Adspeed]

Hello Solidworks experts,
Here's a challange for you!
I'm trying to model a cylinder head for a vintage Harley Davidson motorcycle. Whilst I work with solidworks quite regular this part is quite different from the normal models that I work with as it has a lot of 'organic' geometry so I'm really struggling. I have no experience or training with surfaces so I have been trying to build this model using mostly solid features - maybe this is my mistake. Anyway heres a couple of pic's showing the basics so far (most cooling fins removed for clarity).
Top side:

Underneath:

My main problem I think is how I have created the combustion chamber (cut-loft1 & freeform2 features), so when I shell the top surface away to give me a 9mm chamber wall thickness I get an error about 'minimum radius of curature exceeded' although the shell still completes it creates some unusal overlapping or disjointed geometery which causes me issues later on in the model.
Attached is a SW2013 part file, I would be very happy if one of the experts that lurks here can have a play with it and make some suggestions on how I can improve the shell operation.

 

[rollupswx]

I didn't download and open the file, but you will want to shell this part early in the design - before any of the ribs or bosses.

 

[Adspeed]

I didn't download and open the file, but you will want to shell this part early in the design - before any of the ribs or bosses.

Yes, that is how I have done it. The basic steps were; extrude a solid block, loft cut the chamber shape in underside, then shell the top face. So the shell feature is already as early as is can be.

 

[snowshoe2]

If you have the time try playing with the surface functionality, it's easier than you think and it opens doors that are harder to do with with solids. jump in, you may love it.