Elevation en route was my concern on this as well. What's your high point between Waimea (presumably where you're headed?) and where you're spending the night? I'm assuming it's Kona area, and my recollection is you have a gradual ascent up to Waimea, but it's been 7 years since I drove it....Up in the Parker Ranch area? One consideration is the Saddle Road goes to 6000' somewhere near the observatory turn-off.
I think there's a back way in - inland from the Waikoloa area but I haven't done that in decades. Pretty sure it was all paved.
@diversteve :
Are you referring to the road from Hilo? I think that gets pretty high.