Push rod length will be determined by which head and the valve length. When the heads switched to open chamber, the pushrods got longer. Just be sure to mockup the heads and valve train to check for proper rocker arm to valve tip contact. Also be sure the rocker arm ball is seated on the 7/16" diameter part of the rocker stud.
Adding big block Chevrolet 7/16" rocker studs is wise move. You gain strength and more adjustment room for odd combinations of pushrod and valve length. If you use a poly-lock rocker arm nuts, you most likely will have to shorten each stud some. I setup a fixture in my cut off saw when I shorten them.
Those piston you chose are good replacement pistons. They are how ever brittle and do not like to detonate or take much abuse. When they break, they explode into lots of little bits. A forged piston is the best choice, cast would be my next choice.
Have you called Butler Performance yet? I would bet they can make up a kit for you cheaper then then you can piece it together. I have installed a few of there kits with great success. They come fully ready to install. The balance was perfect, in fact it was the smoothest engine I have ever built, and the machine work was perfect. Every bearing surface was identical in size and spot on.
Joe
