I agree the rear springs are not the best on a f550. I am building one currently, but we also add springs to the top and bottom of the mains. Have been doing this to ALL of our wreckers no matter the brand for the past 62 years. The manufacturers haven't realized where a wrecker carries its weight; behind the rear axle.
The better the rear suspension the less squatting it will do and better steering. You are correct if is on a 60 ca/ with wheel lift at full extension going to have some issues.
I have a 2008 F350 with a 6 ton single line wrecker with sneeker wheel lift. I am towing single axle tractors from rear, 350 box trucks from front and large SUV on wheel lift with no front end issues.