The B1260's design, with the simm going over the CPU, doesn't allow for great cooling of the CPU but it is possible (I cut some of the gills of a coolingplate to be able to fit it under my singlesided simm (still clicks in place)).
The XC runns quite hot as it is, in fact the CS060 MK1 came with active cooling for this reason, so you want to use a coolingplate no matter if you clock it or not. Any kind of overclocking should only be done with active cooling and still I wouldn't go higher than 60Mhz at the most (ok so perhaps you can use 55Mhz without a fan and perhaps you can push it to 66 or even 70Mhz with a fan but it's lifespan is shortened. I would personaly rather have a 10% slower computer for the next 5-10 more years than have it run faaast and see it die in a year or two).