I pull a 60" Bush Hog (brand) 630 pound Rollover Box Blade behind my B3300SU with R4 tires and it works fine in Florida sandy-loam.
L3200 has 500 pounds more frame weight, which will help pulling a wider ROBB.
Do you have 4-WD
?
I added aftermarket Stabilworks pin-adjustable rigid stabilizers after bending the Kubota check chains FIRST TIME pushing dirt with the ROBB. I am
certain you will bend standard Kubota check chains too, with a 72" Gannon.
( My Stabilworks mechs weigh fourteen pounds. Possibly heavier Stabilworks mechs are available from Mark Hodge. Mark produces them one-at-a-time. Kubota check chains were four pounds.)
I am not sure rigid stabilizers will enable you to PUSH looose dirt with a 72" Gannon. You have a lot more traction with the tractor moving forward, less in reverse.
I proceed very gingerly when PUSHING loose dirt with my BH-ROBB, but I do not avoid it. Pushing is a useful mode.
Your are considering an implement right on the border between SUBLIME and IMPOSSIBLE for your
L3200.
How massive is the Gannon you are considering? The whupped Gannon ROBB in the 'Earthcavator' video on YouTube is not that massive......
http://www.youtube.com/watch?v=u6-7NnxGutg
CRIPES, GO FOR IT!