The valve seats in the head have sunk - causing the lifters to collapse to maximum. You need to get the head rebuilt properly. Make sure the valve seats have got a generous margin for sealing the valves - it greatly helps in keeping the valve cool while running. Also make sure that the installed valve height is perfect for your engine. Look it up in a repair manual.
MAKE DOUBLE SURE that the lifters go into the same bores they come out of - mixing them up is an open invitation for a painful life ahead.
Make triple sure the mechanic does not use a magnet to remove the lifters - if he does then your lifters will wear out in very short time - this is because a magnet magnetises them and they attract small metal particles from the oil which acts like perfect emery..
If you need to remove the lifters with the head upright - use a valve grinding suction cup.