I have two 1x8x12 ft treated lumber boards with metal attachment on one end to level with a doorway, which was made as a ramp for a riding lawnmower, to get in and out of a shed. I would like to use it as a ramp for a wheelchair. The raised end is 7.5 inches. Would this be a safe setup for this purpose?
Yes you can make a ramp with 1x8s... this will not bend:
I strongly recommend adding "guide rails" on both sides to prevent the wheelchair from falling off the ramp (in blue):
It's just a bit of wood, say 1" high, on the sides.
The guides are necessary because a wheelchair can't reverse direction in a straight line: the small front wheels have to flip around first. So if he climbs the ramp and then has trouble, or his hands slip on the wheels, or he changes his mind and backs down... The wheelchair will first jerk to the side as the front wheels flip around, then it will roll back down the ramp.
If the ramp is narrow and without "guide rails" on the sides, this jerk when changing direction from forward to back means he will fall off. With guides the front wheels will simply bump into one and get stuck.
So the ramp should be at least the width of the wheelchair plus about 10cm on each side to allow the front wheels to flip around without getting stuck on the guides on the sides. About 80-100cm.
The width of the ramp should cover the full width of the door because you don't want one of the front wheels to miss the ramp when exiting the house.
The picture above is basically what I built for my father when he was in a wheelchair. I used plywood instead, but you get the idea. He had no trouble using it alone. The little guides on the sides were extremely useful, he bumped into them all the time instead of falling off the ramp.
With a wheelchair the weight will be on the wheels which are on the sides of the ramp so you probably don't need an extra support in the middle. However if someone is pushing the wheelchair, they will walk in the center, so an extra support there would be nice.
You could put the planks the other way but this will leave a hole in the middle, and that's really not good for a wheelchair:
...because if a wheel falls off the ramp, even if it's only 7 inches, you can expect the person on the wheelchair to fall off.
While you can leave a full ramp there and everyone can walk on it, if you leave this contraption with the hole in the center and someone exits while being a bit distracted, they'll probably break an ankle or two...