So at this point you really haven't posted enough information to say for sure what sizes will fit your car. The problem is that both your Alter-K and the Heidts rear independent set up change the track width of your car. The body is the same, but without knowing the track width and other suspension clearance issues it's hard to say what will or what won't work for you.
Wracks71 was a member here, he had an Alter-K front suspension and was running 18x9's with 275/35/18's on the front of his Duster with his RMS Alterkation. He said they're from American Muscle, which lists the specs on their FR500 replica's as 18x9's with a +30mm offset, which is a 6.2" backspace. He also said he's running wilwoods on his RMS Alterkation, although I don't know which ones and different brake packages can change the track width a little bit. Based on that information it would seem likely that your choice for the front wheels would probably work, or would at least be close enough to get by with a small spacer. RMS can also provide wheel and tire information for you for their kit.
In the back without knowing the width of the rear axle and any modifications required to fit it we'd just be guessing. Normally a '70 Dart with stock wheel tubs wouldn't be well matched to a 10.5" wide wheel regardless of the rear axle width, as there's usually only about 12" from the inside wall of the wheel well to the quarter lip. A wheel advertised at 10.5" wide is actually 11.5" wide from outside lip to outside lip, the 10.5" measurement is where the tire bead seats. So unless you also have a mini-tub those rims would be too wide. You need about a 1/2" of clearance on each side of the tire, plus the sidewall width that overhangs the rim. Realistically, with 12" from the inner wall to the quarter lip, a 9.5" wide wheel would be about the max for the stock wheel tub.
