Hit a snag

Nice cabinetry work but not such a good design. Not only is design not condusive to air flow but as you have already discovered, difficult to place equipment in. I had a similar design problem with trying to place a large sump under my 90 gallon system. What I ended up doing was to move the rear brace on my tank off to one side. In your case, the only thing you could do would be to cut out a section of the back so you can get you tank in at an angle from the front door if that will fit.

If you try to come in from the back you will have the same problem as coming in from the top. Once the tank is filled on top you will not be able to access the back of the tank (assuming it is up against the wall).

After all this, I think your best bet is to remove the front brace if you can do it without to much damage to the exterior of the cabinet, then refasten it with some kind bracket that you can remove later if needed.