Bumper pulling @ 70/75 is a complete joke, no matter the vehicle.
5th wheel/goose neck with the weight over the rear axle is a trillion times better.
I'll repeat it again, there's a reason the big boys don't bumper trailers. It's just plain not safe or practical, and doesn't scale well with increased weight and length. Too much counter correction is transferred to the trailer because of being hitched beyond the rear axle.
I bumper pulled a 37foot with a modified 1ton, I shorted the frame, use single wheel rear end, and used a short box, as it's my stubby 5th wheel puller, slick for that, but my God it was scary bumper pulling that disaster. Any correction at the front end of truck made the rear of the trailer wag like a dogs tail.
Most miserable 45mph and 1 hour of my life ever behind a steering wheel.
The only exception is a tandem dually loaded with 12 yards of gravel and pulling a 9 ton machine, with probably 4 ton of tongue weight on the pintle hitch.
I did one last weekend. I'll admit, it felt very hitched. It didn't bounce through potholes. It was questionable, because the last time I was loaded like that I snapped a rear spindle on the truck.
Sent from my S-Off'ed HTC One...