there is an issue using that one pce gasket . i dont recall exactly , but remember having to replace it as it didn,t seal on a magnum block with LA front cover. too bad as it is a nice gasket!
The issue is the LA pan has a notch at the front corners and the 1-piece gasket doesn't fill it in. You can cut the "hoops" off the 1-piece gasket and use the 360 LA end gaskets, or fill in the notch with RTV. That's what I did.
Oh, and the 1-piece gasket is for a 360/5.2/5.9 only. The 318 and 340 can't use it because they have a larger radius at the rear of the pan.