apache/cheyenne,
The oil line holes on a '58-'62 261 truck engine are 1/2" NPT (the holes are about 3/4" diameter). This is a full-flow oil filtration system. The earlier systems had smaller lines and were called a "bypass flow" oil filtration system (I think only about 20% of the oil was bypassed to go through the filter).
Do you have the original canister with a cartridge oil filter (with cast aluminum cap/head) or an after-market head with modern spin-on filter?
By the way, there is no need to go overboard with high pressure lines - lines that will handle 120-150 psi should be plenty strong (this is not cutting corners and is less expensive than high-pressure hydraulic-pump-type lines). The main problem with the lines is that they are near the exhaust pipe(s). I understand that the original lines had short (6"-8") metal standoff tubes near the block that transitioned to a flexible line.
Tim
