Use the I-6 alternator on a v8 swap?

Here's a fact - for the later cars, and Jeeps, they would use the GM 10si alternator. So find the parts from a Jeep V8
What V8 are you using? What year? That will be the telling factor. IF an OLD V8, then yes, the brackets are likely for Motorola - but if a 75 or later, then it will have brackets to take the 258 alternator as it should also be 10si.

We need to know the engine source-  the date of the V8, what alternator it had - likely Motorola if it's a V8 older the 74. 
The difference is in the engine age - the year of the car, not in the fact it's a 6 vs 8. 

The one-wire is for lazy, those who don't know better or the details of auto-wiring and charging systems, and for those who don't care about the ALT light, or the fact that they can't just go buy any old voltage regulator and slap it in because - GM NEVER MADE A ONE-WIRE. Those are GM style knock-offs with third party regulators. 
Go get a regulator for a GM from a parts store and put it in that "one-wire" and suddenly you don't have a system that will charge unless you goose the engine RPM up high for a moment. 

You likely didn't get the whole story or the facts from those "handful of other guys" - didn't get the whole story. It's not 6 vs 8 - it's age of the car/engine. 
AMC switched to GM alternators in later years. They used Motorola in the years into the early 70s (except apparently for Canadian cars and those seem to be Prestolite/AM branded).

 Some had Bosch, some had Motorcraft but typical in the early 80s was GM 10si. 

I have a few of them on the shelf from Eagles, Concord, etc.

So find brackets from a LATER V8 and you should be fine. 

You do NOT need a regulator if you use your existing alternator from the 258.
ONLY the cars with the Motorola had the regulator on the fender - when AMC switched to GM alternators they all had internal regulators. 

So if you use the ORIGINAL Concord alternator, ignore those who say you need the V8 regulator. No - it's not ENGINE size or number of cylinders, so ignore that completely. 

ALL you need to be concerned with is the year of the brackets, the year of your car - and using your stock alternator.

In other words, mount your alternator on the V8 and wire it exactly as it was - and you do not need a regulator. 

Anyone who says "v8 cars had a regulator" was referring to the Motorola equipped cars with the fender mounted regulator and Motorola alternator. 

Again - it's not V8 vs 6 - it's YEAR.

As long as you use your alternator - ignore anyone who says you need something else.

The alternator will work the same no matter what engine it's on - even if you put an AMC 287 from a 64 Rambler in your Concord - IF you use your alternator - you can ignore the other stuff. 

Trust me - YOUR alternator will work on any engine and already has a regulator in it. It's a GM 10si. 

You will be fine.

Brackets, belts, etc. are all you need to think about - and you may have that covered already. 

Install and enjoy!

Auto electric is what I do now - and have since I was in the troubleshooting contest in 75.

Not sure what the Alternator Brand and number is that goes with those brackets.

I think this is the same period Jeep Alternator Brackets, but WITH A/C...