I would not even consider cutting cutting the firewall for an access panel. Remove the IP, then anything you can from there. Now remove the seats and control sticks. Law down a board and lots of pillows, blankets, or both, so you have a somewhat comfortable surface to lay on. Use good lighting and while on your back looking up, start removing whatever may be in the way until you get to the GSU-73. Its going to be difficult, uncomfortable and time consuming, but you can do it. Have a helper handy to hand you the tools as you need them. Getting in and out of your working position for tools will greatly add to your fatigue and time getting to the LRU.