Not sure when FoMoCo discontinued the 351W in the US destined Panther but the 351W was available to Canadian buyers for years afterward. You might find it referred to as the 351W HO, but is slightly different fr that offered in the F series fr '84 up. 3 items differentiate the car 351W fr the others. The later car 351W never received EFI; had the 7200 VV carburetor instead. As equipped had good torque for pulling a big car. All car 351W HO's had true dual exhaust fr the factory. All 351W HO's had the D0AZ-A camshaft w/ slightly better lift/duration. This is also the same cam used in the Holley carb'd F series HO offered fr '84 up.

I've owned 2 Grand Marquis having 351w engines. First was an '80 having a 2150 Motorcraft carb. This was not an HO engine; basically just a bigger 302 w/ slightly larger 2V carb & same single exhaust. The second was an '87 Colony Park LS. This was a true HO having the 7200 VV carb, dual exhaust & correct camshaft as evidenced when engine removed & rebuilt. This wagon would easily pull itself, a family of 5 w/ luggage & a tandem axle holiday trailer.

Lastly, ALL pre '92 Panthers had the 8.8 in integral carrier rear axle; also same design (different width) as used in the Mustang GT. The last Ford car to have a 9in was the '79 Lincoln; was restricted to trucks after '79.

I know this is an OLD thread; wanted to input based on original Ford literature I have & my experience. Hope this helps.