I bought the correct number exhaust manifolds for my '70 Bee, same numbers as you quoted(879 and 865), and purchased a 2-1/2" system from TTI. The system fit nicely and sounds awesome. How correct do you want to be? TTI requires manifold numbers when you order. And '70 B and E used the same manifolds.

http://www.mymopar.com/extcastnumb.htm