Have a full size spring mattress that dates back to the sixties. Finding that some springs no longer support weight. Seeking guidance from anyone who might have tried such a project. Much thanks as I await a reply
It's time for a new mattress. Seriously.
You've identified the problem, but neglected to define "the project".
Recommendation, go mattress shopping and replace the outdated, worn out mattress you have.
Mattresses are cheap, and decent ones no longer come with springs. And what exactly are you trying to do (i.e. the "project")?
You might find another use for that old mattress besides still being used in a bedroom. Time to retire that one!