Helenius has a stoppage loss to Duhaupas,points loss to Whyte and a stoppage loss to Washington I’ve no doubt Dubois could KO Helenius. Some good potential match ups for both fighters in the current division but Dubois being younger should be around longer than Helenius .
His handspeed isn't bad, but he's pretty much the same size as Parker and fights like a giant because of his slow feet.
After consideration I think OP is more or less spot on by calling this a 50/50 fight, especially were it to be announced today. However, were we looking at this fight being booked another two or three years from now, then I'd probably have to lean towards Dubois 60/40, as physical deterioration seems to speed up pretty rapidly once a heavyweight is on the wrong side of 40 and Helenius is already 38. Interesting fight though in either case. Dubois vs Kownacki could make for an entertaining fight too IMO.