Yep, as said the 129 manifold won't fit the B engine application regardless of what cylinder head is used, because it is too wide.

The P4876337 could be made to work on the B engine/Stage VI combination, but it is not a 4500 flange like the 129 (not sure how important that was) and as said the integral valley tray will be in the wrong location. It shouldn't be too hard to make it work with a different valley pan but since I've never actually worked with that combination it's hard to offer any advice.

Dean it's too bad Summit/others did not tell you the truth about availability but that seems the norm anymore. Truth is, there are 4 of the 129's in Mopar warehouse stock, and if ordered one could be expected to show up quickly.

We were out of stock on 337s too for a short while but at the moment Mopar/Chrysler does have 3 of them on hand if someone wants to place an order.