Actually, GMs work the whole posterior chain: hammies, hips, low back, and you can even feel your calves pulling. They are no more dangerous for your back than squatting. They are less dangerous and more repeatable than deadlifts, and this is the major reason for their prominence in the original versions of westside: the early days tried to avoid deadlifting. Louie came up with most of the program while suffering bad lumbar injuries. GMs were to protect his back more than deadlifts (and box squats were to avoid louie's troublesome patellar tendon). So the workout is actually designed to lift heavy while minimizing the risk of injury.
The one problem is that the deadlift did suffer. So newer westside has heavy, frequent deadlifting, and the low-back injuries are creeping back in . . .
Suspended good mornings are great training, effective and safe. Enjoy.
The seated good mornings actually take hammies etc out and focus on the lower back. Seated GMs are good, but use sparingly. It's a low back splitter if done to excess.
It's your decision on how much to deadlift. I follow older westside, so the closest I get to deadlifting is rack pulls, or even better, close stance low box squats. I am about 40, so to me, longevity and integrity are worth more than 20-30lbs. As I see it, my maxes will be zero if I am injured.
I also follow old westside on bench, for my shoulders etc. If I did full metal militia training, with heavy shirted benching, board work, and lots of raw benching, I'll tear my rotator cuff again, which was just healed in summer 2003. So I stick to board presses, floor presses, and rack lockouts mostly, and I wear a loose shirt for my "raw" work. Again, I could have 10-20 lbs more on a max with a full-on militia approach, but it may well be my last max of the year.
I'm willing to take the slow road to strength. Live long enough, and you will realize integrity and health are priceless. I have been stronger or weaker at dozens of times in my life. And my numbers now may not be the most impressive of my life, but I still lift. To me, real strength is not a max, but that despite one injury or life-twist after another, I recoup and I lift. Despite yet another tragedy or reversal of fortune in life, I brush the dust off my clothes, stand up and carry on. No matter who or what I lose, I move on. This is real strength of body, mind and character. I will never have my youth back, but I am a survivor, made of iron. This is the strength that comes with age.
Lift for longevity. It's the right choice.