Pushrod lenght: 1st buy or make an adjustable pushrod, set cam so, lets say #1 intake is on the base circle, 2nd install a valve checking spring or no spring at all, 3rd, use a sharpie felt tip pen and color the valve stem black, 4th install your adjustable pushrod, and set the length so the rocker arm roller is just on the exhaust side of center of the valve stem, wiggle the rocker so it makes a mark on the valve stem, now rotate the cam so the valve is full open, once again wiggle the rocker arm to make a mark on the valve stem, you want the marks to be as close to the center of the valve stem as possible when the valve is closed and open. When installing the adjustable pushrod I go for 0 lash on base circle. Don't compress the lifter. Anyway that's how I did it. As far as 6in rods go, the pistons are made for them, you will need to check for the correct deck height when you get all your parts. If you don't have enough deck height clearance, there are many diffferent head gasket configurations and thicknesses. Good luck.