You may want to fit a plate behind the chassis cross member to spread the load, they are normally available from a towing spares place, just take the hitch with you to ensire the holes are aligned, also a good look round the crossmember and where its welded to the chassis, they are prone to rot away on the seams
I dont mean just a look, get into the joints and have a good stab and bang around with a heavy screwdriver and hammer for any rust holes
As far as the actual bolts are concerned, make sure they are the high tensile type, you dont want the same size as on the ordinary 2 point towing hitch. Cause you are spreading the load over 4 anchor points, the bolts are a smaller size