If you want to convert this bike to a gravel bike, your best bet is to go 650b. That is what most folks do when converting an old bike into a gravel mutt. That should let you run at least a 38c tire. Here are two good resources for thinking about a 650b conversion:
https://www.bikeman.com/bikeman-blog...sion-guidlines
https://www.bikeforums.net/classic-v...nversions.html
You might be able to get a pretty wide 700c tire there since the bike came stock with 27 x 1 and 1/4 tires. I own and ride a mid 70s Motobecane Grand Record. It has lots of clearance. Here is the original catalog:
https://www.bikeforums.net/classic-v...nversions.html
If you can get a 700 x 38c tire to fit, then you really don't need to do the 650b conversion.
Are you confident that the BB is French and not Swiss threaded? Also is the rear drop out campy or huret? My guess is campy but if Huret, that will be an issue.