With your idea, you will never have all 650 pounds directly on the "bumper". When on the ramp, some weight will be going to the ground, some to the truck bed, some to the "bumper". Once on the truck bed, 2 wheels are on the bed, and 2 wheels will be on your extension. 650 divided by 2 equals 325 pounds at most on the "bumper". Add the weight of the wood, and you still be OK.
If in doubt, add one more support from the ground to the bottom side of the bumper with a couple 2x4's. Then weight would be no problem at all, or even make that 4 inch extenstion 6 inches, and use a 2x4 support all the way from the ground to the bottom of the plywood? (if you have doubts.). ... Then after it's loaded, simply kick the 2x4 out and toss it in the bed of the truck. This way, there's absolutely NO weight directly on the bumper at all.
Just an idea!