If it is a 1954/55 261, normally it would not have hydraulic lifters (unless the block had been converted/adapted for hydraulic lifters).
Can you post a link to an image of the top of the lifter?
It is very possible that odd/inappropriate parts from later/plentiful 235 car engines were installed over the past 60 years - to keep the engine running. All 235 car engines from 1956 onwards had hydraulic lifters.
If you have hydraulic lifters, you might want to consider buying a set of matching push rods and mechanical lifters.
