The Bell Triton (formerly Sequoia) is also a good helmet for really big heads (if you get the "UL" or Universal Large size). It's the only helmet that really fits me properly but I have the same problem with the chin strap (it just barely fits). My hat size is around 7 7/8-8. I haven't found an Atlas II locally to try out. I have an unused Bell King Head saved in my basement in case they stop making the big helmets. Beyond that, I probably would have to go helmetless as, while I can squeeze into some regular-sized helmets (e.g. Bell Ghisallo, Sweep R), they don't really conform to my head properly and thus are probably of little value in a crash anyway.

As for the chin straps, go to your local mountaineering outfitter (e.g. REI, EMS, etc.). They should have an assortment of webbing and connectors of various sizes so you might be able to rig up a strap extension.