Thats not a very good rule of thumb.....
Cause Horsepower has little to do when it comes to pulling. Old 351's we're only making 150HP, by that rule of thumb a 3.0L Ranger should be able to pull more because its lighter and has the same HP.
If you think a 3.0L Ranger can outpull my 79 F150 with a 351W in it...then bring it on
Torque is what gives you pulling power, as well as gearing. But what you really need is the best Torque\Gear combo.
If your only making 150LBFT@4500RPM then you dont want to have 3.08 gears in the back and try to launch a heavy load......it isnt going to work. But if you have a decent amount of torque, like say 300@1600RPM (351W spec) then you can do ok with 3.08's cause the Engine has the low end torque to get the job done.
Either way....With a 302 I wouldnt go any higher then 5500LBS......with a 351W, 7000.