Magnum going into '73 Dart...which spool mounts? Where to buy?

Not exactly. That pic is of schumacher conversion mounts... from what you say, the previous engine is/was a 318.
So....
LA318, LA360 or 360 Mag or 318 Mag can use the same mounts, use your stock mounts ( replace inserts if needed autozone part number A2369 for insert only )
A2325 and A2326 for the whole mount with insert.
If your mounts look like this you're good to go

LA318->383,400,426,440 : conversion mounts needed
/6 -> any v8 BB or SB: conversion mounts needed ( 2 different kinds depending on if you are using SB or BB )

I'm in the process of a /6 to 360 Mag swap as we type.