If you are using 3 3/4 inch intakes, then you only need one 2 inch exhaust.
Most use the 15 inch fire grill from a 221/2 inch webber for the fire basket, and use expanded metal from 8 to 12 inches high, and make sure it has about 3 inches of clearance from the bottom of the drum.
The lowest grate should be approximately 24 inches from the BOTTOM of the fire basket. If you are using a webber lid for it you can mount another grate above that. It usually comes out to be about 6 or 7 inches from the top lip of the barrel.
That is the tried and true design that works great. Some have modified this design, but for your first one I would recommend the above design.
UDS, cheap o grill, rusted chimney, yeller wannabe thermo pen, Maverick ET 732, 1950 model wife.