I'm trying to avoid you wasting your time,
an important line with a <unique way of identifying> followed by unimportant stuff
a second important line with <another unique identifier> maybe with unimportant stuff
File 2 (parameters)
<first unique identifier> <value to replace>
<second unique identifier> <second value to replace>
Where "<" and ">" are delimiters of any type you choose which are not naturally going to appear in the thing to be acted upon.
Forget it is JCL. Just get the substitution correct. Don't forget it is JCL when designing the parameters/skeleton - can you extend over more than one line with the vols?