They used to rekon with rotavators you need one horse power per inch width of the rotavtor,
That's why as mentioned above you get offset ones of a narrow width to suit tractors of approx 45hp.
A 6ft one is 72 inch so roughly 70 horse power required,
That said I have seen and indeed used a 60 inch one on a mf135 at about 47hp so long as the land is not too heavy, you can compensate with very slow forward speed and not too deep. Make multiple passes if necessary.
What hp is a ford 3910 ?