I have a '95 Ford F-350 4x4 with a 351, and lately (recent 100 miles) I have noticed a very significant decrease in fuel mileage, decrease in power, and a strong smell of unburned fuel. What should I start checking? I was thinking plugs/wires, cap/rotor, and I've gotta new o2 sensor lying around, replace fuel filter, what else is there to replace? Thanks for your help guys!

 

Something to add to the list would be fuel pressure regulator. Pull the vac line off the top. If you see fuel coming out of the vac nipple then it's shot. This can cause the symptoms you speak of.
Make sure to check for proper fuel pressure. Check the specs but is probably 39-40 psi on the fuel rail with the vac line disconnected on the FPR.

Also, before starting, pull the codes. This might give some insight.

Randy