If you get the OEM box and side panel from a parts yard, you will be best to keep the stock 6" sub. The box was designed around the 6" speaker, widening or modifying the OEM box for an 8" is not feasible. The box just fits the 6" speaker, their is not enough material to remove to create an 8" opening. Also, for support, the 6" OEM speaker is threaded and bolted from the rear of the box. An 8" speaker may stress the plastic box in ways that creates other issues, unless you can secure the bottom to the box.
If inclined, scrolling through this link will show what one owner did to create a custom sub box:
viewtopic.php?f=30&t=43846" onclick="window.open(this.href);return false;
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