Anytime I hear talk of a 220 or bigger Conibear on ground level I get rather POed. Yes, you may get the woodchuck; but only if your dog, the neighbor's cat, or your 3 year old nephew doesn't get curious first. A well placed leghold will do the job and is very forgiving if you catch the wrong thing; (care to guess what they use for live trapping or relocating wolves?)
A conibear is forever. It's a great tool in the right application... On the ground in your backyard isn't one of them.
Only if you don't know what you're doing... contrary to popular rhetoric a properly set leghold doesn't do much damage so you can safely release a nontarget species. As I stated before a # 4 leghold is what they use to trap wolves for relocation. I once let loose a beaver caught in a 1 1/2 set... on another occasion my previous dog got caught in a #1 3/4 set for fox, and the only real damage was to the trapper's set.YES a Conibear is a dead on solution, and ALSO WHY I said to COVER IT CORRECTLY! Leg hold traps are not humane and will not kill a woodchuck but it will break it's leg and make it panic and be in a terrible mood for the OP to deal with.
Mark