What's a 3643311 PS gear for?

With the square dome top, that a mid 70's box that should have a large diameter output shaft (pitman spline). Totally interchanges with other large spline 73 and up p/s boxes. Also used in B-series van, IIRC. I think the number casted on the box is just a casting number. Not a part number.

You can use a large spline box on a 68-72 A-body IF you use a 73-76 pitman, centerlink, and idler arm. All three pieces must be used.

I think you can use it on some 72 and down E and B -bodies if you use a certain large spline pitman arm. Don't know details on that one off the top of my head.