Mopar 8.75 posi. wont work. Why are there 2 rows of spline?

MAIN QUESTION. why are there two rows of splines in my 8.75 3.55 posi? and what happens if I don't grab the second row?

So I am a bit new on rear ends and have fought a long hard battle with my new one. So here is the condensed version of my story then i'll get to the current problem. you'll understand why I have to narrate this first two paragraphs once you read the problem.

I have a 1973 Plymouth Duster, it had a 7.25 with 2.76 open gears. For obvious reasons I too wanted the 8.75. One day looking on Craigslist a guy was selling an A-body 8.75 housing for $50. I went to go buy it but before I bought it I found the measurements online and measured the housing I was about to buy. It matched up so I bought it. The very next day a guy listed a 3.55 posi 8.75 center section with new A-body axles and green bearings (basically everything except housing and brakes). Thinking this was a match made in heaven I went to go buy the posi unit/axles. I gave it a good once over and bought it for $400.

Here is where is gets interesting. when I went to go put the two together I found out I had the wrong housing. I used the measurements off that "wrong chart" floating around the internet that everyone curses. So my "right" measurements were actually wrong. The correct length is 52 5/8in (if i remember correct) and I had bought an early 60's B-body that was 53 1/4in. We spend a few days of measuring and calling around and determined that no one could really shorten each side by 1/4 of an inch and still maintain the structural quality of the housing. The Axle's I bought were correct length and had 1in long splines. So everyone came to the conclusion that with a 318 (not stock but still a 318 ) I was okay with only grabbing 5/8 to 3/4 of of the 1 in I needed. SO I had him re-weld the perches, bought new U-bolts and brakes (the works) and tonight I set it down and went to go drive it/

PROBLEM AS IT STANDS:
I jacked it up before I drove it an ensured it did turn with no terrible noise. It seemed good so off I went. It backed out fine and forward (with no gas). But when I went to give it some gas it felt like the tranny was slipping. I checked and indeed it was low I added a quart and tried again. This time I gave it a bit more gas, around 2500 rpm it develops the loud (like LOUD) whining and the car doesn't go any faster. when i pulled it in I put it in park and there was a loud ratcheting sound, like you would get if you tried to throw it into park while still moving...but I wasn't moving. I held the brake and revved it up. My dad confirmed that the DRIVESHAFT WAS TURNING AND THE CAR WAS NOT MOVING. and the whining sound persisted.

DIAGNOSTIC:
I pulled it all apart and the axle slrines were perfect, the yoke and splines were great, and all the pinion and ring gears were excellent. WHAT'S GOING ON!?!? So I found that there are 2 rows of splines in the center section. the first spline is only..what do ya know, 5/8in wide and the second set starts after 3/4in and goes to 1.25In.

MY CONCLUSION:
I am only grabbing the first row of splines on both sides but why does that cause the whining problem and the ratcheting noise? why wont the car drive just fine without the second row engaged. I cannot find any other problem but I figured before I buy a new housing or axles to accommodate the 2 row engagement I would ask you guys.

Sorry for the really long post but I feel it all is relevance to the problem and I couldn't leave one part out without the others. Thanks for your guys time for helping me.