With the added info I would say a few things
- Paulie gave you good info on the 268H, it matches what you want. I also think its a better cam than the 265DEH. I do not understand why you are concerned with the .050 value and compression, but it's not an issue. That's a nice match with the tight quench
- You may need to tweak your distributor with the new cam, so be prepared, you are changing intake valve closing time significantly, and although you are accounting for it with the DCR calculation, it is different and may like a slightly slower curve.
- Your 406 wasn't pinging because of too high of DCR, it was too much initial timing for that motor (or too quick of a curve)
- That 570 will be way down on peak power, and a smaller carb doesn't make more torque, so expect power loss, but if you like the way it cruises and it gets a little better mileage, go for it. Make sure you don't lean it out too much, a lean motor is more likely to rattle. Don't be afraid of the 750 though, when not hammering it, they can get good mileage
Last...
3.50 X .64 = 2.24 final drive.
That is going to be very tough to be where a 410 wants to be all the time. If you are unhappy after the change, consider changing to a 3.89 gear. 11% more mechanical advantage would be significant and would free up power everywhere while still having a 2.49:1 final drive. It would likely even get better mileage. I probably would have done this first
Heck I'd even consider 4.11s, but I assume your tire is pretty short, so first gear may get a little short when horsing around