I'm not sure it would be anything less than a high diff fight. I think it's like the Yonko vs Admiral situation.Harp wins pretty easily.
Edit: Maybe not easily, but he should win this with mid difficult at most. Roger, Garp, WB and Shiki were on another level in their prime.
I think Whitebeard was arguably the weakest Yonko during the MF due to his sickness. The dude could barely use basic Observation and Conquerors Haki.If I did some numbers
Roger 112.5
Whiteboard 110
Shiki 110
Garp 90
Whitebeard (Marineford) 90
Kaido 85
Does that make sense? Even tho WB was awakened, I struggle to have him below prime Garp given the way both him and Sengoku looking at his feats. I don't suppose he should fall behind them.