This project is exhausting....
I see Rockauto has the tailpipes for 66 Darts: Walker
#44168, 273/318s all the way up to 74 same part number. $15.08 + ship
Don't see the 273/318 Y Pipes though. There are some 360 Y pipes on ebay but have different angles up at the heads.
Here is a picture of a 318 Y pipe I got at a swap meet. Angles at the heads are at the same slant for both L & R banks. Might help you locate or make one. Kind of nice the outlet is 2 1/4" OD, 2 1/8" ID (keeping this one)
A Body Tail Pipe, Walker 44168