First off, FCP 7 doesn't mix frame rates correctly, at all. Second, it doesn't edit MP4 footage. Two strikes. MP4, when edited with FCP, will have sync issues galore. if not while cutting, definitely when you export.
Step 1...convert all the footage to ProRes 422. Yes, this will take up a LOT more space than the original footage. Sorry, but that's what needs to happen with FCP 7
Step 2...convert all the frame rates to the one you need to master in WHEN you convert to ProRes. You'll have to do both of these with Compressor...and in the FRAME CONTROLS in the INSPECTOR of Compressor, you need to set all the options to BEST. This means that the conversion will take a long time.
Edit with a more modern NLE that mixes frame rates better, and can even work with MP4...or convert it in the background for you...and work with mixed frame rates. FCX and Adobe Premiere do this....just letting you know.
You are 2/3 of the way done editing? Meaning you already mixed the formats? Well, if you want it to look good, and not skippy, you need to stop...convert the footage, MANUALLY recut the 2/3rds you already cut as you cannot simply relink, the frame rate and timecode will be different. And then continue editing to finish.