Would this be a bad idea?

Here is the Background on the car first. 1971 Swinger 360LA , has a very mild cam ,but I don’t know the specs. RPM intake ,Holley carb, headers, cast iron heads with factory shaft mounted rockers. I have owned the car since 2016 and put about 5000 miles on it. Previous owner said the motor came from eBay and was rebuilt. For the most part it has been mostly trouble free given the unknows with a 50-year-old car. My one gripe with the motor has always been some valvetrain noise that makes it sound like a sewing machine under acceleration. At idle the noise is not as noticeable. The motor runs and performs good ,I just feel things could be better, so today I did some reading up on valve lash. I followed this http://www.hughesengines.com/Upload/productInstructions/ValveAdjust.pdf. What I found was two rocker arms on separate cylinders that have what I would describe as the absences of any preload. I performed the procedure with the motor cold. Now to the question. Could I try a slightly longer pushrod to add some preload? What is the likelihood I have two wiped cam lobes? wouldn’t the motor run like crap? Thanks for the help, this site is a Gods send.