I will be putting it in the B+ line with the key activating it.
It is a 12 volt, 105 amp alternator. I'm running #6 cable.
In order to even begin to answer, it would be helpful to understand what you are trying to do with this relay.
If I understand correctly, you are using the key switch to activate the relay which would either connect or disconnect (i.e., interrupt) the B+. This appears to indicate that there would be the possibility of interrupting the AO (alternator output) when the alternator is operating. This is not a good thing.
The only thing that comes to mind as to why you may be doing this is to de-energize the AO circuit from the house bank to the alternator when the engine is off, automatically.
If that guess is correct, you would be much better off by placing a manual switch in that wire, as Maine Sail and I have been suggesting for many years. Trying to "automate" that function makes little sense, since a shutoff is only needed when you're working on the engine anyway. It is so rarely used on my boat that I simply remove the fuse, while I have a switch I never installed it!
You would also have to consider whether it would be a NO or NC relay, and it would most likely need to be continuous duty. NO if power failed to it means like the key switch off. But then, if a wire to it failed, it would open and interrupt your AO. Not so good. NC would alleviate the AO issue, but not work for my guessed intention.
If my guess as to why you want to do this is right, I think it's a bad idea.
Also, a #6 wire for a 105 alternator sounds awfully small.
