IIRC, the early models mount differently than the 77-78 ones, so that might explain what you see on youtube.
I sent you pics via email of my setup -- it looks like the muffler mount is actually in a similar spot, but like I said I had to use brackets it came with labelled for some other bikes. I do have the longer muffler though, but it looks like the 2 bolts are in the same spot.
First and foremost though, did you already tighten the headers to the head? Because that could be your problem -- you've gotta leave them loose while you fit other things, because they aren't a positive fit like modern bikes/stock exhausts.
Also, while you shouldn't need to adjust THAT much, the kit comes with milled aluminum spacers to move things around if needed.
All that to say, 'plug and play' means something a little different for 40 year old bikes than modern ones, I think
