We had the same problem with a 340 build. Several sets of lifters later for the customer and still ticks. Found the new lifter diameter was smaller. This combined with worn lifter bores created low pressure to the lifter.
Now we check every lifter and bush the block on many if worn. Most are worn. These engines are 50 years old. We have been buying old mopar performance cam kits for the lifter. Even if we don't use that cam we use the lifters. Mopar and Pontiac all take the same Hyd. lifter