I was working on designing an exhaust manifold, but while trying to combine the different bodies I created I get basically two (2) errors.

I was trying to shell the cylindrical faces and I know I needed to combine the bodies to do that.

These are the errors
1. Rebuild errors - Unable to create single body that is the sum of the input bodies.

2. Has to do with geometric relations.

I attached the part file below. Please help.