http://www.cactiguide.com/forum/viewtopic.php?t=2498 <- Has photos of Stef.
Stef is very tall and thin - recently he fell over and I was wondering if it's safe to cut a section from the top of him to balance him up?
I have 30 cacti and I have never had a problem with them as they are small - I just haven't had one as big as him and he's my favourite.
Stef is top heavy. :(
If you cut a section from the top then the remaining bit will sprout arms. That is likely to make it even more unbalanced in time. But then you can root the section you cut off and grow a more balanced plant. Try to give full sun, the plant will put on more width and less height, much more able to support itself.