Tom,
It sounds like you have a pretty good idea of what you want to achieve and have figured out how to get there. The book you referred to is probably the one by W. Ellis Brown, he has a table in it for reference that gets us close to where we need to be, from there on you just have to tweek it a bit more. The one in 70 twist rate on the fully rifled barrel should not effect the shot spread much, it will be like having spreader loads in it, good for Grouse or Quail.
I have read some place, a test study where they compared the slug shooting accuracy or several slug barrels, some fully rifled and some that just had screw in rifled choke tubes. The winner of the test was a fully rifled tube, but the screw in chokes came in very close right behind it. It would not have made much difference at deer shooting distances. The main point of the test was accuracy, but they also mentioned and stressed that matching the ammo to a particular barrel was the only way to get best accuracy out of any certain barrel.
If you can find, or develop, a load that does equally well in both your barrels, then you have just about got it figured out. I would concentrate on accuracy alone and forget about speed, whatever load you finish with will have enough energy to do what you want. Bob