As a tow vehicle, it doesn't rely so much on the power, as it does the RATED towing capacity of the vehicle.
I have an MX83 Cressida with an 1UZ V8, has bags of power and torque to do the job, but Toyota DO NOT under any circumstances recommend towing more than 1500kg and therefore the trailer and car total mass can't exceed this to be legal & the highest capacity tow bar is made by Hayman Reese and it has a 1500kg rating as the MAXIMUM towing rating for the car is 1500kg.
Check the ratings for the R31 before you enter into this conversion as you might just find it a waste of time for the purpose you plan it for.
Just as a reference, you have to be caught for it to be illegal, but if you are & your involved in a fatal accident, your going to GAOL, think about this when your planning this conversion. And another thing, and I don't know how they get around it, the max capacity motor that can be put into a vehicle is 3 times (3 X) the body weight and I would be surprised to find the R31 of any description would weigh 1500kg dry.
Cheers, D