As you may know, I have a 70's Mercier, well the cottered cranks died on a century on monday... bolts fell out of the chainrings and bent the chainring out. also the bearings were mysteriously loose after the incident so i think the derailer bending blow was pretty rough on it!
(you see, french threads are close but not exactly like british threads, and they're like italian in that they are both right hand threaded.)
I was not about to be defeated however and took apart a spare bike I had (90's rockhopper, 72mm bb, iso square taper), got the sealed bottom bracket and got it installed tightly in my bike and all was well! It fit perfectly with the bb cups that were original to the bike and centered too! but while I had it out I played around with the cups and noticed that I could thread in the british/Iso right hand thread cup into my frame about 8 turns by hand... no matter, I had my bb secured with the french cups.

Then I realized that my cranks were not really what I wanted and I bought a set of FSA Gossamer cranks with ISO bb included. WHOOPS! Get the cranks and realized it wasnt ISO, it was Splined ISIS! craaaaap! Isis splines do NOT fit through the old cottered crank cups, so I was back at square 1. I decided to use 2 right hand Euro cups, and an extra british lock ring from the cartridge bottom brackets and they threaded in all the way, without too much hassle. here's what I did:

1. I cut a vertical line in the french fixed cup to use as a tap for the threads and cleaned them out very thoroughly.
2. I got another lock ring off of the french and hooked it in a vise, then I used the bb cup tool and ran the cups through it with plenty of grease once, then I left the lock ring on the very very end of the cup.
3. I measured the bb to find out how far to screw in the drive side cup and lock ring, and double checked chainline.
4. I put grease in the shell and on the cups! (Don't forget this part!)
5. I screwed the Right hand thread cup all the way, on the drive side and tightened the lock ring, just like the old fixed cup. (Use the british lock ring for this side for extra strength!) This part is where the threads stretch, and to be honest it took a little force, but the fist 5-6 turns were by hand so I trusted it. the Bottom bracket got warm, thats how you know its working!
I gently turned in a half turn, out a 1/4 turn, in a half, out a 1/4. It took less than 5 minutes a side! you can see on the new cups the threads were easily morphed to french thread and there werent any metal shavings!
7. insert the sealed bb cartridge, and screw in the other side, make sure you leave the lock ring on the very outside of the cup because it will not go on if you forget!
8. Tighten the lock ring.
9. Install cranks
10. ride

I took everything apart a few times to make sure it was all Kosher, and i'm really surprised but it is solid! I can easily screw in the british/iso/squished threads ISIS cups by hand all the way with no drama. I'm so happy! no more french cranks, threads etc!
The good part is I can always go back to stock because the old 70's steel was a heck of a lot harder than the new stuff and the theads didn't morph.

Why didn't you just get a french BB?

English threads are 34.8mm, French are 35mm, so all you've really done is put a slightly smaller bolt in a bigger hole, and probably buggered up the threads a bit. But remember, it's a little small and that may make it come loose more easily. But what you've done has been done by others - and has been reported to "work".

FYI, for future reference, Harris has old-skool french BB cups:
https://harriscyclery.net/itemdetails.cfm?ID=3204

John, when you get my bike you will find an interesting BB in there!

bike is supposedly italian frame, it had cottered cranks (might still have them)
in 1979 I bought a new set of sugino maxy , well the supplied bb cups would not fit- oh my...I was 13 and it was alot of money for me, We ended up using the bb cups that came with the frame, and the sugino axle.
hey it worked.

I have not looked seriously at that bike in decades.

A good quality Isis BB for french threads....

Like I said I took it all apart a few times to see what I'd done and the cups threads shifted, the frame threading is the same (the old cups go in and out easily)

If it were a square taper I'd have gone on VO, but I dind't want to spend $50 and have to find different cranks. There's a reason its purported to work, it does work. haha, It's because the newer metals used for cups are soft to avoid stripping out a very expensive frame, the old steel frame is very hard.

Also, the British cups are a slightly larger diameter, so there was room for it to lengthen the cup, toothpaste tube style.

British cups are 34.8mm (1.37"). French are 35mm. Ya got it backwards

Hmmm, I just find an old French TA or Stronglight BB on ebay... I think I used one of the VO ones too. I hate buggered up stuff when you could just do it right in the first place.

comparing them side by side, the french cup was smaller. I didn't want on old stronglight or TA, I like the steel frame but the cassettes and chainwheels without grooved/pinned teeth are too much for me to handle.