Sorry so late to the game. Haven't checked the forum in awhile.
I ran into the same thing, wanting to increase the T-1661 bunk capacity as the kids got bigger.
So 2 things I meant to do, only accomplished 1 for now. I built two 2x4 and particleboard frames that would span the bunk supports and set in place. I slid them in through the fire exit window and finaggled them till they dropped in place in the folded out bunk "tray". This alone made a big difference, only drawback was you couldn't fold the bunk back up.
Other thing I wanted to do was stiffen the side supports with some steel angle iron, attached to both the supports and into the wall studs. Haven't got to that one yet.