Both of those '72 Swingers are really nice, guys. Love that body style.
IMHO, do the build all at once for the reasons stated above.
But really, you need to answer to yourself as to what you're really looking for out of the build in order to engineer it to meet your goals.
You may not even need to stroke it to get the numbers you might be looking for.
FWIW, that 360 build in the December issue of Mopar Muscle is next on my agenda for my '73 Dart Sport.
Build A 500HP Capable 360 Magnum Short-Block On The Cheap! - Hot Rod Network
I'll be using a '73 LA 360 block rather than a magnum, since I have a clean std. bore one here already, but basically doing much the same as in that article.
I figure 450 hp out to the 3:55s in the back should be plenty for that car.