When I built the 408 for my rock crawler dodge, I had to have custom pushrods made. I used a company called "Trend Performance" and got them through Summit. They where chrome moly, and had options for ball or socketed ends, different diameters, and most any other option a pushrod could have. The way I got the correct length was by cutting an original pushrod in two, threading the two pieces that I just cut inside the oil hole, then installed a long threaded stud. This made an adjustable pushrod that I could use to find out what length pushrod I needed. Afterward I marked the top of the valve with a blue marker, then adjusted the pushrod length after turning over the engine by hand and checking the rocker sweep pattern on top of the valve. Sweep pattern is a smidge important as far as getting the geometry correct to not wear a valve guide out. This really only changes if you have different height from original valves, deeper set seats, or rocker arms that don't match the head that you are using. If you have to go this route, don t forget to factor in the lifter preload if you happen to be using hydraulic lifters.
