rotisserie problem

i bought an eastwood 3000 lb rotisserie and made brackets to attach where the bumpers were bolted on, the front seems to be ok but when I started jacking up the back it started to bend the brackets and seemed to lean forward so much that all the weight was on the single front wheel and the 2 back wheels were actually off the ground so I redid the brackets and mounted them uderneath on to the frame rails and it still seems to be leaning forward , not as much but I can clearly see it pointing down , I have looked at other ones on this website and another one they were both bolted on at the bumper location , one was a duster and the other a barracuda notchback. I am stumped ,I am using the attaching arms that came with it and it can't weigh more than the 3000 lb limit because the drivetrain and rear end and springs have been removed , any help would be appreaciated Thanks