Solid lifters are fine. The flat tappets do require the zinc and phosphorous levels to be in the 1,200PPM to 1,500PPM to aid lifter and cam lobe life.
Used to hear poorly maintained slants drive by with the tappets singing like a drunk band, and they would still go 200k miles. Once broken in and the parts "like eachother" annual adjustment is likely OK, depending on the amount driven and spring loads. For the first 10,000 miles or so the lash should be checked occasionally until they stay set.
