I want roller lifters and rocker arms on my 350 motor I have decided on a Lunati cam and lifter kit, to replace the Comp Cam flat tappet set up the previous owner installed. my question is will a have to get new push rods because I know I need new springs seeing as though the current cam is a 400 lift and the new will be a 500+

Yes you will need new push rods, but not necessarily because of the springs.

Roller lifters are usually about .400" taller than flat lifters, meaning you need push rods about .400" shorter more or less.

You order pushrods of specific length last, because you have to measure accurately to get the valvetrain geometry correct. Everything else has to be bolted together and in place in order to measure. This is one of the more complex procedures in setting up a valvetrain.