Adjustable rockers are good . But you can always shim your shafts to get the correct preload. Zero to 1/4 turn with adjustables. One important thing is clearance the rockers for the retainers being used. All 340 rockers are clearanced from the factory for the performance springs. 318 and 360 2bbl were the same rocker but required less clearance and usually not done on some. Same with iron adjustables from early 273's always check clearance