Chain laws are covered in DOT rules 393.100 to 393.136
I do not know your state laws so I will use federal laws.
Federal law says you need 2 chains for equipment less than 10,000 lbs. Some states say you need 4. Four seperate chains and binders is not a bad idea.
You need to tie down with a downward force of 20% of the weight. Just make the chains very tight and this will be covered.
You need to chain the rear to hold 0.8 g deceleration in the forward direction; This means you need 6000 lbs X .8 = 4800 lbs of chain on the rear. Two 4700 lb chains will be over kill. Heck 2 grade 70 1/4" chains rated at 3150 lbs would be enough.
You need to chain the front to hold 0.5 g acceleration in the rearward direction; and 0.5 g acceleration in a lateral direction.
That is 6000 lbs X .5 = 3000 lbs of chain rating.
One chain would be enough if you use 5/16 grade 70.
The trick with using 1 chain on each end is holding lateral movement.
So lets say you get 4 chains (5/16 grade 70) and binders. One on each corner. You now have 4700 lbs X 2 of hold down on each end (9400 lbs). You only need .8 on the rear so you could haul a tractor over 11,500 lbs with those 4 chains.
11,500 X .8 = 9200 lbs.