Hold the pump in your hand,and push the arm against a workbench or some such stout surface, actuating the arm,while holding a thumb over one of the ports at a time. The one you feel vacuum on is the inlet.
The arm is just of a slightly different design. The end of the arm still lands on the camshaft in either case. It's just different geometry. The new one will work fine.
Almost positive it was 5/16 originally. But why not use a 3/8 pick up and line forward in case you up the HP later. I did it on my GTS. I doubt anyone will look under the carand take points off for it.
Yup, I love Holley DP's. Used to build many of them for my buddies back in our hay days. Still will use an AFB or AVS on a numbers/show restoration. But much prefer the Holley.
I can deal with and or rebuild and tune any brand. But I do prefer Holley, as I believe, they are of a simpler design, and have found them to work better in race applications.
I've got a 750 cfm Marine Carter that I had on my Stroker Engine in my Wellcraft Boat. I have used on several different Mopars and Chebbies with no issues. I do think that I jetted it bigger for the Stroker.