I just bought a 1978 DL Wagon 4wd and it has 13 inch wheels. I found some 14-inch Peugeot wheels (4x140) and thought I would mount some slightly taller tires on those - and switch to all terrain tires. Will this cause any issues? How big can I go without problems?

Thanks in advance.

 

Nice looking wagon. I don't think you will be able to find a 14" tire at stock height that isn't a passenger snow tire. What size depends on offset and availability, but either way there is not much room in those wheel wells.
K

4Wheeler magazine (or similar) did a mini-series on their project Subaru.  I think they did swap in 14-in. wheels, but could only fit a marginally taller tire (no lift kit).  You may not even be able to find a larger-than-stock tire in those wheel sizes anymore...