Hallo Geri,
Here also in France we have some regulations for the tires to use.
On my 3600, with the 15-6 rims, the maximum size of tires allowed is 195R15 (195/80-15) and this is what I had on my truck. I have now changed the size of the rims to 16-7 and have 225/75-16 tires on them. The look is better and the dimension is close to the 7.00-15 originally installed.
For rims and tires in Germany, there is
http://www.oldtimer-reifen.com/ which offer a good range of rims and tires for vintage vehicles.
If you want some 195R15 tires, contact me, I still have some ;-))
Michel from France