I have a pump casing that is cast in 2 parts Xpress Doesn't work for assemblies.

I have couple of question on simulation Xpress

1. The Fixture faces for bolts. Can I get away with split line to create the washer faces. I am using these to simulate the casing being mechanically held together by bolt pre load. The bolts are stiffer and  and have higher yeild strength than the casing.

2. The casing is mounted to a non-rigid body is material is  
stiffer than the casing is it bad practice  
to used the mounting faces as fixture?

3. How do you find the critial failure areas with out sectioning the part. as express won't  section. something to do with the XYZ co-ordinates in the .out file?

Material for casing  Cast A351 CF8M
Material for bolts   A4-80
Material for pedestal Cast A352 LCB steel

I would be grateful for any advice on FEA simulation of pump casing.