One way to shape your trim is to bend it around your smaller tank. When it springs back a little, it should be about the right size for your larger tank. The width is really your preference of appearance. 1" by 1/8 th would be adequate, unless your cuts are really nasty, then I would go with 1 1/2" by 1/8 th. You really don't need 3/16, it just adds extra weight. Even if you put a counter weight on the door, you might want to be mindful of the overall weight because it all adds up. If this is going to be a stationary cooker, who cares, but if it will be a mobile pit, all of the extra weight comes into play with moving it by hand or towing it long distances. When trailer mounted, keep in mind that you will be hauling wood, charcoal, coolers, tables, canopies, cooking tools, etc.
Also, Rubbin Butts has a good point about the safety issue with you firebox location. You might need to put an expanded steel guard near there to keep your leg safe.