I've been working on removing the dash out of my 80 z28, and am working on the heater core. I've removed the two bolts on the inside & the top of the heater box is loose. I've removed the passenger inner wheel fender & I see the stud with the nut attached under the lower heater hose. However, I cannot get the nut off. Everytime I try to turn it with a wrench or impact, the stud turns with the nut, possibly due to being rusted, I'm not sure. I've tried holding the stud so it doesn't turn with vice grips while i turn the nut with a wrench but I cannot get the stud to stop turning with the nut. I've attached some pictures of the two bolts I've removed on the inside, as well as the stud/nut that keep turning below the lower heater hose. Any help or advice would be appreciated. Thanks!