Don't know exactly, but it would be somewhere close to 8000 lb.
On edit, I just looked at the specs for the '79 Burbs. It has to be 4WD, since you have the 400SB. GVWR could be from 6800 to 8400 lb, depending on the exact springs and wheel tire combo provided with the vehicle. Since yours has the 400, it is likely close to the 8400 max. Only way to be sure was obliterated when someone painted over it.