A person with the ability to bend and contort and twist a 5"/5"/8' steel beam. This person is strictly super. I understand the person has to have similar or greater density. In this mock-up, the persons bones are stronger by far then the muscles. the strength is proportional in every way throughout the body, but he looks like an 18 year old, who is lanky and somewhat thin. He is 5'9". What would be the diet for a person like this? Muscle mass does have a bearing on strength. What could he do in his peak, if he trained? And lastly, would he be able to stop a full semi-truck with trailer in tow with his body if position right?
My comment notwithstanding, assaying an answer here:
What would be the diet for a person like this?
The person's diet would be broadly in proportion to the energy expended. If most of what they were doing was human-normal, they'd probably consume calories equivalent to a normal human. However, as their flesh and bone would have to be a density similar to metal, just moving around would use substantially more energy, so it's safe to say they'd be a big eater.
Their stomach would also have to be magical, because there's no (demonstrated) biological way to turn a diet of earth-sourced foodstuffs into something that looks like flesh but is many times its density and toughness.
They would also break most things they sit in, as they'd weigh at least 500kg.
What could he do in his peak, if he trained?
That's more or less up to you. Muscle (as we understand it) cannot be as dense as you've described, so this isn't muscle as we understand it. If it's analagous to human muscle, he could be about eight times as strong. The average, untrained man can bench-press (as one measure of strength) 137 lbs. The current world record for bench press is (with equipment to support the muscles) 1105 lb, or approximately eight times the force. This is an extraordinary individual, though - if your superhuman is typical of whatever he/she is, probably no more than six times the strength is achievable.
And lastly, would he be able to stop a full semi-truck with trailer in tow with his body if position right?
No. A full semi-truck with trailer can (legally) weigh up to 40 tons (80 000 lbs). Your superhuman (probably) masses about half a ton. He would do some damage to the grille, but at speed, the semi would still punt him out of the way. If he were buried up to his knees in asphalt like a bollard, he might stand a chance of stopping the semi, assuming his tensile strength was increased in the same vein as his strength and density, but if he's standing on the surface, there's no position where the incident friction would be enough to stop the truck.
Edit: As an example, a Mini Cooper's curb weight is ~1300kg, and this happened.