Too many posts!
I've driven a 4CV with a 330 gearbox and it certainly is a nice conversion. A friend in France gave up on his 318 4-speed and installed the 3-speed all synchro box from later Dauphines - with a high diff ratio it goes well but with a big ratio gap. Knowing Andrew's hills I'd be looking for 2 intermediate gears, that is for sure. Most of the 318 cluster shafts that appear here are damaged in the first gear. Old, stored gearboxes can be perfect if the car died for a different reason!!The rear transmission plate issue is a bit murky because of the different versions of various gearboxes. The 314 gearbox (1959-1961 Dauphine and 4cv) had both the thick back plate (up to sometime in 1960) and the thin steel one. I think this is also true for the 318 four speed box. Don't use the 318 gearbox!! The primary shaft eventually fails - I have never seen a good one. Use the 330 gearbox with the steel back plate from a late 314, 318 or a 325 - I think they will fit the gearbox. If not, the plate could be modified. Dauphines sold in the USA beginning in 1964 came with the 330. I eventually changed my automatic Dauphine to a 330 four speed. Since the Dauphine engine bolts right up, the 4cv should also. I have not modified a 4cv cross member, but I think that is the way to go if you do not want to cut the body. Here is a picture of a 330 in a 4cv that sold recently. It doesn't show the crossmember, but shows that it is possible.
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It also seems that there are two sets of intermediate gear ratios on the 318 gearboxes. I've no idea which is which but the information is out there no doubt in the factory parts books. That means you have to be careful using a cluster shaft that isn't from the same gearbox when rebuilding.