Russell Day long, Bill Mayer, Laam etc. can all make a saddle that can be adjusted to fit you. RDL seats tend to push most riders up but a phone call to them can verify if they can lower you. They can move you up, back, down and all around to make the profile what you want. You could also run some lower profile tires to drop the bike a little. Personally I'd shy away from lowering the bike in any way be it suspension mods or tires because the cornering clearance is already low enough on these bikes. I'd go with pullback risers to start with to get you sitting ack into the bike. If you can't reach, try a seat that moves you forward an inch or so. The bike is more narrow as you move closer to the tank and you might pick up a little reach there as well.

You need the bike to fit. 5'8" isn't short by any means but if you have a taller torso and shorter legs, you might need a different bike. The GL series bikes are about the largest bikes on the road and they aren't for everyone no matter how many mods you add to make it work. That being said, I once saw a barely 5'0 tall woman on a GL1800 and she handled the bike like a pro so it's not the skill that counts here, It's the fit. There are smaller motorcycles to fit smaller riders. I'm 6'0" and these bikes are almost too big for me. It's just a thought you could consider, not directing anything harsh or negative in any way. Not everyone fits every bike, that's why they come with so many different sizes and options.

I agree with olegoat on recommending the RDL even though he has a Corbin in his picture ;-)