Any gains adding 1.6 rockers to stock 273?

It's not going to be a night and day difference.
Before doing anything, I would make sure the rest of the motor is up to snuff with everything working the way it should. Compression and leakdown test, optimize the timing and advance curves. If it's a high mile engine, you may need to open it up to change the timing chain anyhow, and at that point it's not much more work to switch out the cam. They're fairly peppy when running right.
Is it a 2 bbl. or the 4 bbl. HiPo motor?
If you do decide to try the rockers, SpeedMaster has 1.6s for about $200.
https://www.speedmaster79.com/Mopar-Chrysler-SB-318-340-360-160-Ratio-Stainless-Steel-Roller-Rocker-Arm-Set
They're not top of the line, but more than adequate for a stock 273. Just be aware that with these and many other rocker sets, you'll also need different pushrods since the factory used ball and cup pushrods and these use ball and ball.