Be sure to check the
documentation for each of these options to have an understanding of what they do.
In this situation, the best way to fix the problem isn't actually with NCrunch configuration. If one of your projects can only run within an x64 environment, it is then structurally important that any project depending on this project is also set to target x64. In this way, there is no way your code can be run outside of the x64 environment.
The best option is to go into the VS build settings of each of your projects that rely on this project (including your test projects), and set this to target the x64 platform. NCrunch will detect this and adapt accordingly.
If you're looking to just force NCrunch to use x64 for your test project, set the 'Use CPU Architecture' setting on the test project to x64.