So just how good was the A-Body back in its day?

Here's my personal experience:
My 1st Mopar was a '69 340 Swinger that I bought used in November 1969. It was just a couple of months old but the owner was going into the Navy and his parents wouldn't keep the payments up for him. I paid $2500 for it. Beautiful car, Charger Red with a white butt stripe, and Cragars already on it.
All my friends had either Chevys or Fords and laughed at my Dart - until they couldn't keep up with the 340 and then their tune changed.
One of my friends in college had an early Barracuda and went to test drive the 340 when it first came out a year or so earlier. Came back to school the next day and described it as a "Jr. Varsity Hemi".
I lived not too far from Whittier Blvd. here in SoCal at the time and regularly cruised it on Friday and Saturday nights and loved beating up on big-block Chevelles with my small-block Dart. It was lighter and quicker from the start and could walk away from them from a stoplight - and street races didn't last for a full quarter mile so the big-blocks didn't get a chance to get into their horsepower advantage.
Also used to run G/PureStock class at OCIR on weekends and did well both in class and in brackets. One of my early races was against a '68 Roadrunner who was also running the same class but with a couple of "cheat" things that Tech hadn't caught. We both ended up in the final round that night. Before the race he came over, looked my Dart over and sneered, "It's not even going to be close."
I got him off the light and could see him creeping up on me as got close to the finish line - didn't see either win light flash as we went by. It wasn't until I picked up my slip with the "WIN" stamped on it that I knew I'd beat him.
We were pitted fairly close to each other and he had to drive by to get to his pit. As he was coming by my wife stepped out in front of him to stop him, walked up to his open window and said, "You're right, it wasn't even close".
Best time I ran in G/PureStock was 14.28 @ 98 mph. 3.23 open rearend, no headers, just a good tune. -
I've been in love with A-bodies ever since.
As far as engineering, my '69 340 had a forged steel crank, floating wrist pins, and shaft-mounted rockers from the factory. It was really easy to add a dual-point distributor and forged adjustable rocker arms - other factory parts easily found at the salvage yards. The 727 Torqueflite behind it was the best automatic trans at the time and the 8.75 rearend was almost bulletproof behind a smallblock. Throw an adjustable pinion snubber on the rearend and you didn't need any of the multiple add-on traction aids needed by the Chevys and Fords of the day. And Chrysler's factory race parts program was top-of-the-line.