magnum r/t springs
There is no Comp Cam spring that is comparable to the R/T spring. The stock spring is a beehive spring and the retainers for those springs won't work with the R/T spring, the 2.2/2.5 4cyl retainer is the correct size. Comp Cams has a beehive that fits the magnum head without machining but it will require new retainers too.
I would say you do need new springs.
The factory springs are barely adequate for the stock magnum engine cam shafts. The smallest Voodoo cam has .454 lift with 1.5 rockers. The magnum head uses 1.6 ratio rockers so you will wind up with .484 lift. Plus the Voodoo series cams has very fast ramps which requires a stronger spring to control compared to a more typical ramp design with the same lift and the 1.6 ratio rockers makes the issue worse.