Its funny you should be thinking of an F4 conversion from a Frenchie, because its just what I am undertaking. However, in reality I am just switching a french van body onto a donor rt hand saloon. To be honest I was a little rushed in getting a van. I would avoid North France, because, even though its from France, the van chassis isn't in as good a condition as the saloon ( I was lucky there). The donor gives you all the bits you need, the important thing is to ensure the a good body and a good chassis to start with. The other advantage has been an increase in engine size, although the original engine, 849cc, was extremely nippy, but it was not under "load".
There are some minor modifications we will have to do to the van body, such as a new hole for the steering shaft, and decide whether or not to fit the van handbrake, or the saloon version. It would also be nice to have the rear door hinged on the right, rather than the left, so it opens onto the pavement, but that looks a little too difficult. The finished van has to go back to work to earn its keep. At present I am trying to get my sandblaster going to remove three coats of paint. It was originally mid blue, then it joined the army and became completely olive green inside and out, then on de-mob somebody has given it a coat of cream paint, possibly Dulux.