Typically, they use filler- about 50 little dots. Some also weld in plates to cover all the holes not used and filler over the joints. Don't forget to cut the center crossmember out and make it a bolt-in, along with the e-brake bracket, while the engine's out. Then, the oil pan can be removed while the engine remains in the car. Doing this with the engine in place can be done but its much more difficult, as is installing a ss firewall cover. Think about it.
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