What lifters are you using? Are they stock replacement lifters? OEM? I believe those lifters have a pretty large range of plunger movement. From my experience running more preload can quiet down the valve train a little. At some point though, it probably becomes paralysis by analysis. Sounds like your method was correct. If you want to run more preload, it as simple as screwing in an adjuster and locking it down.
I'm curious - did you have any clearance issues with the 3/8" pushrods on the trick flow heads?