The two small terminals nearer the centre of the rear of the alternator are field terminals. Don't hook anything up to these except field wires (one of these terminals gets grounded if you're running a pre-'70 style regulator). The big threaded-stud terminal is the B+, or output terminal. It is electrically common with the battery's positive terminal. No problem using this for a power feed. But it is live all the time, not just when the key is switched on. It sounds as if your relay might not be hooked up correctly; make sure its #86 terminal is connected to a switched +12v source, that is only live when the key is on. #85 should be grounded, #30 should go via a fuse to B+ (alternator output terminal is fine), and #87 goes to the module/coil power terminals.
If your relay #86 presently connects to an always-live +12v source, it'll drain the battery overnight.
Also, careful with that 90A alternator in an unmodified stock charging system. Standard ~12ga charging wire and an unmodified ammeter aren't a good mix with a high-amp alternator. You'll read all kinds of scare stories about how the ammeter is a ticking time bomb just a-waitin' to burn your car to a crisp. Wrong. The stock ammeter is perfectly adequate for stock charging systems...especially if the connections are cleaned and tightened once every couple of decades whether they need it or not. Throw a high-output alternator into an otherwise-stock system, and yeah, things are going to burn up as soon as you hit a high-charge condition (battery goes bad, or needs topped up after extended cranking or a couple of hours with the lights left on, etc.). That's not because the stock system is "dangerous", it's because tossing a high-output alternator into a stock system is the wrong way to do it. The '73-'76 A-bodies with heated backglass got 65-amp alternators and 8-gauge main charging cables separate from the rest of the engine wiring harness...right from the factory. And yet guys will toss big alternators in their cars without changing anything else, and then blame the predictable result on the stock system being "dangerous".
Go take a look at R/T Engineering. They can rework stock ammeters to handle up to 70 amps efficiently, and they can convert stock ammeters into voltmeters (complete with a new gauge face that looks like factory equipment). Their website's got a bunch of really good info on it, but you have to mouse around awhile to find it; the organisation isn't completely obvious. For example, I know there's a pic of one of their voltmeter conversions somewhere on the site, but I can't find it at the moment! Be sure to see this page. They actually have intelligent, Mopar-owning and -driving people who actually answer the phone, so if you get fed up trying to find what you want on the site, call them. I've got one of their upgraded ammeters waiting to go into my '73.
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