The Z/28 pan has 2 strips of steel spot welded inside the pan that mate up with the curled edges of the windage tray. The strips will seal off the tray sides. You will notice the fit when you try to install the pan - you have to draw the pan tight with the screws as the windage tray assy will hold the pan away from the bottom of the block slightly.
The pan is still available from GM Performance Parts in the catalog. You can download the catalog and find it. It might be $135 or so but I have not bougt one for some time. The only functional difference between the production pan and the current service replacement is the front seal thickness. The current service replacement uses the '75 and up oil pan gasket setup (as do all current service replacements for older pans). You will need the 5 special studs with stand-offs, 5 prevailing torque nuts and the ~$35 tray.
You might want to get the high pressure spring for the stock volume oil pump if you need it for a whopping $4.