Power is not as much of a problem as braking!  I once towed a Westy with a 4 cylinder Isuzu Trooper II.  I could accelerate OK (The combo still can almost out accelerate a Westy!), and the brakes were up to the job (Nice 4 wheel disk brakes on the Trooper), but the limiting factor was the "back end trying to pass the front end" problem in a stop.  I also made a turn too tight coming out of a parking lot and ran the Westy over a curb (at very low speed) and it set up an oscillation that felt like it was going to flip the trooper over!  There is no substitue for a heavy tow vehicle.

Bill
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 I towed a westy from Moline to Detroit a couple of years back with an Ford
 Explorer towing a u-haul tow dolly. To book, I had to say that the Explorer
 was a V8 version and not V6 - something to do with u-haul weight/power
 combinations which I could not get to the bottom of so just 'played the
 game' - The guy I picked the tow dolly up from couldn't care less as it
 turned out but the guys on the reservation line seemed particular on
 details.