If lower back is the primary concern then heavy rack pulls will do more to strengthen the low back than really anything else, since the loads you can use with rack pulls greatly exceeds what you could use with deads or deficit deads. This is assuming that you are using your back to lift the load with rack pulls and not squatting the bar up as some people are known to do. I honestly think that rack pulls, which I incorporated early on, are pretty much the primary reason that my erectors are as strong as they are. I remember a few months after I first started rack pulling and I felt my erectors and was blown away. I hadn't reached back there to really feel them since prior to starting rack pulls and the difference in growth was tremendous.
You may want to look into straps for heavy rack pulls if you decide to go that route, just so that grip is not the limiting factor. To focus on lower back strength, I would stick to doubles or triples on the rack pulls and only do a few sets. 9 reps total would probably be sufficient at first. How much are you rack pulling right now? When my max dead was only 405, I was rack pulling in the 600s after getting accustomed to the lift. I wouldn't be surprised if you could work up to a 500 lb rack pull.
If you did do this, I would drop the load on deads back for sure. Keep them around the 3 x 5 x 315 range while you focus on strengthening up that lower back. Or even just do the rack pulls on deadlift day instead of deads.