Jelavic scored twice in their thrashing of the Welsh side, whilst Long was a constant thorn in the side of the Bluebirds' backline.
With both players having joined the Tigers in January, Bruce has revealed his delight at how quickly the duo have settled in.
"It was a strange afternoon but I've said for a long, long time that strikers make the difference. Today, Long and Jelavic in particular have got the goals," he told Sky Sports.
"At this level, it's so vitally important because we know how difficult it is. They've only been at the club for three or four weeks and already Jelavic has three and Longy has a couple which helps overall.
"It's difficult for a team that have just come out of the Championship and to put in the work ethic that we require - both of them have it in abundance. As you've seen today, they're both very, very good players.
"It's good for (Jelavic) because he could have had six or eight since he's been here. Like every striker, you need a goal. Today, he took his goal fantastically well and led the line terrifically well."
Despite back-to-back away wins, Bruce insists that Premier League safety is still their No.1 priority and refuses to be drawn in by talk of a top-half finish.
"You have to be difficult to beat at this level and of course, rely on your strikers to get something away from home," he said.
"We've had two great results - Sunderland away and this one today.
"Everybody's still in it, there's still a lot to play for with 11 games to play. (West Ham manager) Sam Allardyce has won four on the spin and look where it's propelled them. It's not over yet."