You'll have to swap the dashes, and the dash wiring harnesses in the dash and under the hood (it's all attached to the windshield cowling and fenders). From the B-pillars back, wiring will the the same until the taillights.
BTW - the frames are not quite the same, as that E350 will have taller and thicker frame rails. But, the bodies are the same and mount the same.
You might want to figure out how your state handles body swaps. If you have to register is using the '99 VIN, then have to pass emissions testing, there's no way the 460 will pass the requirements that the '99 meets.
2000 E450 dually V10 wagon