careful examination of this pic
would show that if you mount the 5.25 fan adapter/hdd cage on the bottom most 5.25 bays it would still benefit the CPU portion of the overall cooling schema. the top of the adapter does not exceed the height of the PSU. however if you leave the fans above the PSU and the roof fan running. the resulting low pressure of those fans will simply redirect the airflow upwards where there are NO hot components to cool. you want the low pressure generated by the 2 rear exhaust behind the CPU to be the main exit points for airflow so it will carry the hot air out of the case.
if you look at your existing HDD cage, it's ENTIRELY blocking the cool airflow from the front intake fans from the TITANS which are 265w cards each. you are looking at 795w worth of heat that NEEDS cooling air and you are blocking part of that cool air feed with a huge brick of HDD's...
yes you do have 2 fans on the side panel but it's supplying air towards the exhaust end of the card. the intake side of the card is still towards the front of the case which the bottom front fan is supposed to supply. besides, considering the amount of heat the titans are putting out, and considering you are already using LOW CFM (small)fans in the 1st place it's a good practice to use all 4 to supply cool air for the GPU's. the 5.25 fan adaptor/cage is simply supplying additional air to help the CPU out.
this is your case, and you are going to do what you feel works best. but i'm just letting you know that you shouldn't underestimate the amount of heat that triple titans can put out and the existing fans you have on the case was NOT designed to cool down 3 x 265w cards. when that case was originally built. there was no such thing as 265w cards
we are talking about the beginning of the pentium4/GeForce2 with a whopping 32 megs of video RAM
era back in 2000
here is an example of the state of the art video card your case was engineered for
if you were cooling a SINGLE card like this (there was only 1 AGP slot on the mobo
) then blocking the front intake fans with bricks of HDD wasn't a big deal. but for triple titans.... you may want to reconsider what is blocking your cool air intake and maybe remove any obstacles between the cool air and the titans
your titans hits 70C idle
... my GPU hits about 70C while i'm gaming... this means you are TOTALLY wasting your titans by not supplying it with enough cooling air... most people who has enough money to invest in 3xTitans have enough sense to get a case that can supply enough cooling to allow the titans to breath. i'm not saying your case is not a good one, but DO REALIZE it was NOT
designed to handle the amount of heat you are subjecting the case under and steps are necessary to allow a case of that age to take on the thermal challenges of today's GPU's.