I recommend using Exorcist III footage of the Karras tombstone in the cemetary (making sure to avoid the people) and Photoshopping Lamont’s name over Karras’. And to put Regan in there, either find another film featuring Blair or someone like Blair, and cut ’em together, making sure to color their clothing black, probably, if necessary. OR, render in an over-the-shoulder shot, where it’s apparent it’s Regan, and you’re golden. The simpler your goals, the easier it’ll be. 🙂 Good luck.
M.I.K.E.