If you want to use stock type rocker arms ( I won't ) your going to have to use custom length pushrods to get the preload you want on the lifter cups. Don't be surprised if you need several different length pushrods to get all of the preload exactly the same We use to have to buy and use adjustable pushrods on all our builds that didn't come with adjustable rocker arms on the motor to start with to use solid lifters
The other option is adjustable rockers with the proper length custom pushrod