Vocal/guitar Bleeding ... Try this:
I would strongly suggest you did NOT use any kind of collar on the mic. Using a collar like that will strongly effect both the freq resonse of the mic and it's pickup pattern.
Polar patterns are interesting things ... Most people don't realize that if you block off the botton half of the ball on a cardiod mic (such as a 58), the pickup pattern quickly approaches omni. The vents from the rear on any directional mic are critical to it's proper use. Don't fool with them. =)
Also, for most players a divider between the voice and the guitar would be (as someone said) awkward, and I could see it really taking away from the performance. The people I've worked with doing guitar and vox together were already a little put off just being in the studio ... The more crap you stick in front of them, the more apprehensive they become. That's been my experience.
The best bet is to mic vox and gtr as closely as possible without getting boomy. Then point the mics away from the other source as ,uch as you can.
The setup that I live by (at the moment, anyway) for simultaneous guitar and vox is this:
1 C1000s on the neck end of the guitar, pointed in towards the body a bit and at roughly 45 degrees downward, away from the performers mouth.
Another C1000s on the other end, pointed towards where the performers pick or fingers are hitting the strings. This one is also roughly 45 degrees downward.
Record these 2 in stereo.
For the vox, a good large diaphragm condenser of choice, put as close the singer as possible (use a pop filter!) and pointed slightly upwards toward the mouth.
Tweak to taste.
This a very simple yet VERY effective setup. There are many many other setups I'm sure, but this is truly one that works.
Of course that's just my opinion. I could be wrong.