I'm having the same issue with my 71 w108. The rear deck cutouts are closest to a 4x10 but more like 4.25x8.? Weird metric size, but oval not round. The best thing I have heard is to get adaptors. For mine I can use a 6x9 to 4x10 adaptor. It won't be a perfect fit but can work without cutting up my rear deck. You should be able to do the same with round adaptors for your application. I don't think it's a good idea to go cutting up that steel, deflates the value and kinda hard to do. As far as your deck speakers interfering with your sub, like someone said, I don't see a problem.

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