I can answer that question! Dealing with that right now on my 383. In order to get reasonable preload, not only did I need custom pushrods, but they needed to be 2 different lengths. Since the pushrods tend to be within 0.005" tolerance of each other, I optimized the lengths and now have preload of 0.035" to 0.044" = right where I wanted to be.

Real pain and if I had adjustable rockers you just turn the adjuster screw instead of 2 different length pushrods.