Engine won't start after swap

I just installed a new long block into a '99 XJ. Fired it up without hooking up radiator or exhaust just to ensure that it fired and it started right up. Installed radiator and went to check for leaks and now it wont start. Nothing changed from before radiator was installed. Is it possible to kill the O2 sensors if they are just hanging under the chassis rather than installed in the exhaust?
The engine swap has depleted all funds so throwing parts at it is not an option. The CPS is fairly new and I do not think it is the problem.