Is this my original 71 frame?

[dfairchild19]

I am trying to figure out if the frame under my 71 is original. There are several things that make me question if it is. First, without even looking at the frame, the grill is off of an earlier first gen. The body mount brackets are bolted on instead of riveted on. The rear springs appear to be 1 ton springs and im not sure that they offered a 1 ton first gen?? It also had a body lift on it as if they were trying to clear the frame or something.

But then there are other things that make it appear original such as the closed knuckle front diff, the first gen transfer case cross member(??), and some brackets that still have their original rivets.

I dont think it was converted to 4x4 because i am almost positive that the front spring hangers are still riveted on. My thoughts are that it could be a 73 to 87 frame or that it could even be original with some of the rivets replaced with bolts.

Here are some pictures, Ill start a build thread soon but i need to know what gen frame i have to order 4" lift springs so i can get my 14b/d60 under there.
thanks in advance