I never actually used the migration tool to install the 707 into P3D, I only copied the files from FSX to P3Dv2/v3. Seeing as I bought the migration tool years ago, I can give it a try to install 707 into P3Dv3.4 (I doubt the 707 will work in P3Dv4).
Seeing as the migration tool will install the 707 in P3D, just like it would be installed in FSX, it's worth noting that to get it working without the problem of the fps droping over time, to do the fix that Mustang1977 provided here: https://www.captainsim.org/forum/csf.pl?num=1463049439/14#14
Sadly, that means you won't be able to do flights using dopller navigation, only using GPS, or INS navigation flights. :(

I will do a test of installing the 707 into P3Dv3, using the migration tool, and let you know how it  goes.

[edit]The migration tool worked fine for installing the 707 into P3Dv3, and the 707 worked like it should when I used it. I still had to comment out the DNS gauge in the panel.cfg to prevent the frame rates dropping from 50+ fps, down to ~7 fps (it only took a few minutes to drop down to that).[/edit]