Correct pushrod length is determined by a lot of factors, including block deck height, head deck height, rocker arm brand/design, lifter brand/design, valve stem length, and other factors.
Whenever you are using aftermarket components in a cam swap, you are going to HAVE to measure correct pushrod length for proper valvetrain geometry. You need to ensure that the tip of the rocker stays on the valve tip, and rolls across it smoothly from the intake side of the tip at low lift to the exhaust side of the tip at max lift, and that it is centered on the valve tip at mid lift.
No one can tell you what length you need. Everyone's setup is different. Even the type of head gasket used (thickness) can mean that one guy with the same heads, cam, and rockers as you will need a different pushrod length than you will.
Agreed, there are too many variables. Improper valve geometry usually causes problems in the future, do it right the first time that way you won't have to do it again when it breaks.