His side are chasing the League Leaders’ Shield while their opponents at a sold-out Leigh Sports Village, whom they have beaten twice already this term, are currently eighth.
But Powell said: “When you look at the balance we’ve beaten them twice but things have flipped a little bit since then.
“Widnes’ form is pretty good and we’ve lost three experienced players in Justin Carney and Weller Hauraki to bans and Grant Millington due to injury.
“Kevin Brown is playing so well, too. He’s an outstanding player and probably the form half in the competition and tough to manage.
“We’ll need to play outstandingly well to win the game. Obviously we’re a confident group – we’ve been confident all year – but for me they would start as slight favourites.”
Though then playing for Sheffield Eagles, a 20-year-old Powell was stood on the Wembley terraces in his Castleford shirt when they last lifted the trophy in 1986.
“It’d be great to replicate that but we’ve got to get this job done before we do any other,” he said.
“That’s the key – not to get too far ahead. The players have done a great job in taking it one game at a time and this is the ultimate for that; if you don’t get this one right you are done and dusted.
“We’ve a great chance to do something special but it’s about actions now and how they manage themselves and our game.”
James Clare will take over from suspended winger Justin Carney tomorrow.
Andy Lynch and Daryl Clark are set to return after being rested last week, but Tigers are still without Grant Millington (knee) and Weller Hauraki, who is suspended.