Deacon,
One of the things the Germans wanted to with the Marder Conversion program was to take advantage of having lots of Pak 40 ammo and lots of captured Russian 76.2 mm guns.
The Germans rechambered the russian gun to fire the 75 mm round.
You can get nice rounds from Verlinden and a number of other manufacturers.
Aber also sells a set of shell ends... that work very well for filling in the ammo racks.
Hope this helps,
Tim