Is it new, if yes....tight, no slack.
You are not going to get any further until you get a stock pushrod
to get started, at least to see where you are.
I think you were told earlier a roller pushrod is not going to be right.
The other way is to buy an adjustable pushrod
which will find the right length for you to use.
Then go to SummitRacing
and look for a set of pushrods of that length.
Pushrod search at Summit.
For the SBC engine, there are "off the shelf" pushrods in any length you might need. They come in 0.050" increments.