chrisscruton,
There is not a lot of room there, but there may be enough, depending on what you want to put on that tire as far as chains go.
Like MadRef, I have 2-link ladder chains on the R4s on the back of my B2910. I think I may even have less clearance than you do, but they work fine if you keep them tight, and cut off the extra links on the inside chain.
Attached is a picture. I kept my chains on all year of 2004, due to all the rain we got. Put 350 hours on the tractor without any problems with the chains at all.
These chains are the "standard" kind that use 3/8 chain links.
Don't write off the possibility of using chains on those tires without spacers. It may work if you keep the chains tight.
On the B2910, turf tires are wider than the R4s. and reports are it is impossible to use chains on turf tires for this reason, unless spacers are installed.