Need suggestions - reinstalling Y drain fitting between back and grey tanks

[therink]

I just completed removal of the valves and 3" Y drain fitting on my black and grey tanks to remove debris from my black tank valve seat. My grey and black tanks are about 18" apart and the common 3" drain pipe enters up into the belly to a 3" Y fitting between the tanks. The drain pipe, Y fitting, and both grey and black tank gate valve flanges are one single piece (glued). I need to find a way to separate the tanks about 3/4" while they are in place to get the valve flanges to pop back into the tanks while keeping the o rings in place. This is not fun to do when there is residual tank water dripping in your face.
Anyway, the tank side walls have some flex and am thinking of gently lodging a couple 2 x 4's in between the tanks to push the side walls in just enough to get the flange lips to pop into place while being able to keep the o rings seated. I do not want to cut or replace the Y fitting and close nipples if I don't have to.
Anyone have suggestions?
Steve