Training dog to guard

I have a 1 year old 110 lb german shepherd cross who has been guarding my poultry. He was raised with the chickens and accepts them. OK, I’ll admit he does like to pester the roosters into chasing him, but he leaves all the hens alone. Anyway, I recently lost a few pullets and found out that it was a raccoon. I would like to give him a little more formal training to teach him to giuve chase to raccoons/fox/possums/etc rather than just bark at them. He does chase cats like crazy and kills groundhogs he gets ahold of. So, I think he has the drive needed to do this. I am, however, a little hesitant to tap into the aggressive side of a german shepherd and am trying to be very careful. I had a german shepherd before that overguarded and was scary with stangers (esp strangers that weren’t confident around big dogs), and don’t want to end up in the same situation again. My previous german shepherd was a purebred and out of police dogs lines, so is probably the best comparison. Any tips out there on how to teach my dog to go after “critters” without turning him into a dangerous dog? I suppose I thought his natural guarding tendancies would just guide his behavior… It does guide his barking, but if I want anything more than that I think I need to teach him. He can be taught to do almost anything (even some “weird” stuff), and taught rather quickly, but he does have to be taught. Other german shepherds I have been around are generally this way too. It makes you be careful about what you teach them, because they will actually do what they are trained to do and do it without hesitation…