I have a new set of 980-12's ... if your running any boost don't bother with the 979's .. your talking roughly 10 pounds on the seat difference
after a few heat cycles your probably right at stock levels
You are fine with putting 980’s on stock cam and lifters. Your lifters are hardened and broken in already. I wouldn’t put less spring than 980’s. It just wouldn’t be worth the time it takes to do it. Many of us put LT1 100# springs in back in the 90’s without any issues. As long as you realize flat tappet cams require good oil that’s high in zinc and don’t try to run mobile 1 or other thin synthetic oil you will be fine.