Rear frame crossmember - leaf spring truck

[MidLifer]

I am thinking of putting a 31 gallon 87-91 Blazer tank into my 71. Doing this will require moving the rear frame crossmember that currently meets up with the rear leaf spring mounts.

By moving this member back to almost the end of the frame rails does this introduce any potential frame weakness or flex issues with leaf spring trucks? I tow trailers and put heavy stuff in the bed. It's a 2WD so I'm not off-roading except for the occasional NJ pothole ;-)