@Just BS- my offset started out as just a grill made out of a 55 gallon drum so there's a charcoal grate below the cook grate, and that's where I put the firebrick. They act as a sort of a tuning plate and keep flames from lapping up from the firebox and scorching the meat. I suppose you could lay some along the bottom of the drum if you don't have a grate down there, or use some flat stock to elevate them just below your food grate. And yeah they're pretty much end to end with about a 4" space opposite the firebox and along the sides as well. Dryer vent will work for exhaust as long as it's the aluminum flex kind... or use a beercan with the ends cut off lol! Hope that helps.
UDS'-55&16, Akorn, Bar-B-Q Kettle, Drum Grill, miniWSM, cobweb-covered gasser. I'm JD.