OP, how did you set the adjustment preload on the lifters? I'm almost positive that none of the 273 motors came with hydraulic lifters stock, so that means the pushrods you used are for solid lifters, not hydraulics There to long probally All of the OEM hydraulic lifters, both early and late, have a different cup hieght, higher, than the same year solid liftrs cup hieght so the push rods your using are probally to long collapsing the hydraulic lifters The 340 T/A, AAR Cuda motors had a special set of pushrods to make them work with hydraulic lifters, not the same lengths as the 273 solid lifter pushrods Let us know what you find out