If you're using aluminum instead of steel rule of thumb is to make it half the original thickness again. 3/8 plate steel would be replaced with 9/16 aluminum. Use a good alloy whether it's steel or ally. CRC 5.56 is a really good cutting
oil for aluminum or kerosene is the old school solution. You can cut a lot of metal with a cutoff wheel in a 4" grinder, I like Pferd wheels as they give a good balance between cutting and wearing. A sawzall would also work well on plate like that.