The whole machine's custom built to fit inside the cylindrical case... the best you could do would be to take the outer case off, and then you've just got a slightly smaller cylinder.

Electrically, everything's built around a round "central" PCB using a custom interconnect. You're not going to be able to reassemble the thing into a rectangle and still get a functioning machine (not without tons of custom design work, at least).

See https://www.ifixit.com/Teardown/Mac+Pro+Late+2013+Teardown/2...