You have posted an excellent question and I will try and answer it as best I can, based on what I have found while researching the same question. Simply put, the standard factory weapons arrangement for A-2 and A-7 aircraft would be two wingroot MG151's, two MG151's in the forward ventral gun pod (the outer two blast tubes in the front would be open) and two MK108's in the fuselage/upwards firing arrangement. That would be your safest arrangement to base your build on. With that said, what modifications in weapons arrangements were made by crews in the field would be up to you, based on photos of the particular aircraft you are building. The ventral gun pods were still built to accept the two rear guns and could be installed later. The upwards firing cannons could also be removed and the muzzle ports on the upper fuselage taped/faired over. I would be surprised if any planes had more than six guns, in any arrangement, based on wieght considerations and what I have found.
I believe the photo that shows the four blast tubes open is mislabeled and not WNr. 310189. I have a found a couple of photos showing this plane only had the two outer tubes open. It would have had the upwards firing cannons installed at the factory, but if they were removed and the ports taped/faired over I can't tell from the photos I have seen.
I hope this helps and I can try and post or email you photos if you want.
Damon