Yup the mini chassis is a 700 footprint but as mentioned it's the bolt handle cut out that will be the biggest different. Easy fix with a little dremel work and a reseal. 5 minute job.
The ejection port shouldn't require any mods as the Defiance is probably cut longer than the 700 ejection port. Below is a pic of a 700 set up Manners with mini chassis with a TL3 dropped in. You can see the ejection port is longer on the TL3 but it still functions perfectly. You could trim that small area if needed for OCD looks but not needed so going from a custom to a 700 you should be fine in that area.
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