Extending Surface Bodies with complex geometry

[HarrisonClassic]

Hi, attached are three bodies that essentially, I want to use a cutting/trim tools between each other.

What I but in order to do so I need them to be about 40mm larger on all edges with natural extension.

I can't seem to work out a way of doing this - be very interested in ideas on this workflow.

thanks

Dave

[davebYYPCU]

Works for Tangential, and a better outcome, 

Natural failed on all attempts.

Might help

[HarrisonClassic]

I think the strategy might have to be some sketches on multiple construction planes. Projecting the intersect of the face, extending those lines and creating lofted surfaces to stitch to the original….