nice tank,
i have built one very similar for my project, mine is 4.25 id (4.5 od with 1/8 inch thick walls) and 10 inches long, i did the math and that equals only about 2.3 quarts total volume, i dont think that you would want to top it off so mine might hold a max of 2 quarts, i am considering running a oil cooler to make up for some lost volume and to help with cooling.
how much open space do you have at the top of your tank after you put oil in it? any problems with over flow or over heating?