There's a little bit of room for play with the width of those types of stands... It might say 51", but that's with the cross members forced as far into the support brackets as they will go. wiggle them out 1/4" on each side, and you just bought yourself an extra 1/2"...

The only problem I can see is that the G70 is a VERY heavy keyboard, and unless you have it balanced exactly on the balance point of the bar, it's going to put considerable torque onto the joints, which are plastic and have been known to strip or break.

One thing I noticed is that there is a support bar on the back as well. Maybe that could be used to support the rear of the G70?

Anyway, given Sweetwater's great return policy, I don't see a downside in getting it and if it does pinch a bit too tight, you can always return it...

Do you intend to use this at home or in a studio as a permanent installation, or do you intend to gig with it?

One final thought... Have you tried these kinds of stands? I always found them quite bouncy, especially with heavier keyboards.