hydraulic lifter problems

they are flat tappet lifters the cam is thexe275hl gross lift is 525 with the 1.5 rockers 561 with the 1.6 rockers they are cast pistons i have 4.10 gears and 26" tires with the old lifters it would not make it to the traps i had to let of the gas at at about the 1000 foot mark i dont want to rev any mor than i have to if you bring it up slow it will go to 6400 ithink i can get thru around 6100 but that is when it starts to break down i went from the ld340 to the tm5 it revs quicker now on the dyno the ld fell off around 5000 the heads are ported i think it should be good to 6200 but with this cam and 1.6 rockers the valvetrain may be unstable