Alright, so here is the update from ARP (two emails actually):

1) my question was: are the materials different in these two bolt kits?

"There is no difference in the material type of the two kit numbers you have provided. They are both the same chrome moly steel. The only difference will be the collar diameter. The exhaust manifold kit has a reduced collar diameter for clearance on the header tubes. If this will not affect you, then you should be good to go."

2) my question was: so what is the actual collar diameter on the 1" bolt, or at least what's the difference between the two of them (so that I can measure out the clearance I need on the headers)?

144-1202 UHL .750” Collar OD .530”
641-1000 UHL 1.000” Collar OD .620”

Bottom line, the 641-1000 is just fine given that the washer that comes in the 144-1202 kit is already a 0.670" side anyways.