They are all close in stature to be honest- Hearns- 6'1 1/2" with a 78" reach and fought as high as 190Lbs. while still in decent shape shape. Hill- 6' 1/2" with a 77" reach and fought as high as 200Lbs while still in decent shape. Kessler- 6'1" with a 73" and is ripped at 168Lbs. I'm sure he would be comfortable at 175Lbs. With that said, a prime Hill still beats Kessler rather easily... as would Hearns.
Hearns was still a welterweight champion. 20 pounds below where Kessler fights. Hill would easily outbox Kessler. If you look at his resume he beat some good fighters like Czyz and Stewart and Tate and Tiozzo.
I think Kessler would be more comfortable at 175 than 168, he looked a fair bit bigger than Calzaghe when they fought. Either way, Hill on points in a pretty passive affair, 117-111 or thereabouts.