The B body top cowl is taller than an E body. The vertical portion is similar, but shorter on E bodies, so you can use a repro E body panel and cut it down as a repair panel.
To fix this, you need to remove the dash, heater, pedals, firewall insulation and all that stuff. These cars rust in this spot because the upper cowl and the firewall are designed in such a way that they are supposed to drain water, but unfortunately they were bare metal in that area, and had a foam sealer between the panels, which causes the welded joint to rust all along it. This means the firewall rusts through and water leaks into the car, making the floors wet too. There is a little wall around the vents that rusts away too, so usually those need to be rebuilt or replaced. Mice love to build nests in there. It all adds up to lots of hidden rust