Blue Oval

whats a good size TB to put on a 351w thats making around 340hp at the crank. something with room for improvement if i ever get a set of heads in the future. im not sure what size the TB is now so would i measure it on the air filter side or the upper intake side?

Edit: also where do you measure the MAF to find out the size, but i think its stamped on the bottom right?