It won't much matter where you put the diffuser in that size tank so long as the filter is moving some water. I wouldn't put it in the HOB... I think it will be a little too shallow to get my absorption, and some will offgas as it spills out.
FWIW, I have 1 tank with the diffuser right next to my intake, and one that is on the opposite side of the tank. Both CO2 systems run at about the same flow rate, and both drop checkers always read green after some minor adjusting to start.
Do you have 4dKH standard for your dc? If not, I will ship you some along with the ferts.