Question on shelf setup

[mdhunt71]

Hey all, I am building a cabinet that is 72" in length, 24" depth. It has a 24" blind side that is covered up by another base cabinet. 
The shelf that was "built" by the software is 70-3/8", which gives me 1/8" for clearance of the shelf. 
My issue is that I can't seem to find a support piece for the shelf in the cut list for the cabinet. Being that long, I don't think it would handle being supported by shelf pins on the side. 
I would think it needs a piece or two of plywood screwed in underneath it, as well as a support piece under the middle of it.
Any thoughts or suggestions?
Thanks
   

[admin]

The blind bases have an option for a partition but the shelf will still be given a full length that will need to be cut to size.

[mdhunt71]

The blind bases have an option for a partition but the shelf will still be given a full length that will need to be cut to size.

Ok, thanks for the clarification.  I have another question now. haha
The back pieces are all roughly 4-1/2" short in height leaving an opening at the top of the back of the cabinet.
I am fairly new to cabinet making myself. Is this a standard thing?
Thank you