Sorry to hear it's not all to plan. A head can appear to "warp" if the deck is not trued and the head is not the original, or has been milled. Uneven thermal stresses (ie, clogged passages in the head) could contribute to distortion. Uneven bolting is another contributor.
The part about "lifters being too weak" has me puzzled. The pushrods may be too short to get adequate preload, or there is/was an oiling issue - the cam lobes are worn or the lifters not oiling (lack of their rotation can be a culprit - see point about cam wear). Either way, the cam and valve springs should be matched to not over stress the valve train. Overspringing is wated power.
No matter what, the head has to come off. Check your deck surface for true, check your head again; mill if it's feasible. Measure the cam lift with a dial gauge on all lobes before doing anything to the valve train, and make sure the springs are suited to the job, not overkill.
Regards, Adam.
