Replacing header gasket on amc 360
I had some exhaust work done on my '77 today, and when I picked it up, the mechanic showed me where I had a leak on one of the headers. It's on the drivers side, around cylinder #5. He pointed out the dark areas around the covers, and I could feel and hear the leaking exhaust. He said replacing the gasket is real easy; He thought I could just back the bolts out some and drop it in there. I've ordered the gasket from Oreilly, but I haven't seen it yet, and the salesman asked if if was supposed to be "U" shaped? I would think I would have to remove the exhaust manifold to get a new gasket in. I've looked through my Haynes manual, but they don't have any steps to doing this. Can someone give me a brief idea of how to do it easiest, and any tips would be helpful.
He also asked me, again, what year the motor was. Since I've never been able to find anything on the motor to tell me that, I have no idea. I told him I figured any gasket from an AMC 360 would work; he found a '78 Matador in the computer with that size motor. Am I on the right track?