“I made a couple of changes and put on three quarters of a stone to a stone to try and be a bit bigger.
“I felt like I got a bit manhandled and I never want that again. I just felt it was men against boys at certain points, especially for me personally, and I didn’t enjoy that.
“I feel a lot stronger in my carries and still move pretty well. It was something I decided to switch up and so far it’s going pretty well for me.”
Wakefield, who are attempting to reach a first Challenge Cup semi-final since 2016, welcome a Wigan side who had enjoyed a flawless start to the campaign before suffering back-to-back defeats by Huddersfield Giants and St Helens.
Johnstone said Trinity will be prepared for a top-level showing from the 21-time Challenge Cup winners.
“They’re a massive club and they’ve had a lot of success over the years and they’re always there or thereabouts,” he added.
“It’s a bit uncharacteristic of them to lose two on the bounce, but we’re expecting nothing but the best Wigan to turn up on Sunday and we’re going to have to be at our best to progress.
“That’s what you’ve got to do if you want to win a competition, you’ve got to beat the best teams in it.”
