I couldn't find the L4960 on Tractordata.com, but it might be close - I have all older stuff (like me), and have a 7' Gannon (not ROBB, has hydraulic rippers) - my old Allis 160 is likely a bit heavier than yours
www.tractordata.com
and i have almost 600 pounds of front weights on it so (most of the time) when I raise the box (full) it USUALLY raises, INSTEAD of the front wheels coming off the ground - also, mine's 2 wheel drive and I have to use the differential lock about half the time to keep from just spinning in place...
I've not used a ROBB, but seem to recall people saying they're a bit more to pull than equal sized regular box blades. My Gannon weighs just under 1000 lbs empty, probably around 2500 with a full box (depending on material, of course)
All I got, HTH... Steve