I completely agree with Clemsonfor. Save someone's life: stop using boards to load that tractor. Make sure you buy or build ramps that drop into something or have pins or some other positive retention. The 186D especially is deceptively heavy. It is a good to put some blocking under the rear of the trailer as well.
Even ramps rated for the load will fold on you. I barely escaped when a cheap, lightweight set of ramps supposedly rated for 4500 lbs gave way under my tiny Kubota B6000. It very nearly killed me, and is by far the closest call I have ever had on equipment, in spite of doing a multitude of foolish things over my lifetime.
I have since deliberately destroyed and disposed of the remaining ramp, and replaced them with a set I built. These new ramps are essentially indestructible with my equipment. It is a nuisance to deal with heavy ramps, but not as inconvenient as dying or being injured will be.