Dear Dominic;
The additional line (from filter) was probably put there because the rockers wern't oiling from below, through the 'inside' line etc.
When things are all apart & cleaned, just check the flow through the line and oil passages in both the block & head.
Also; The line has enough oil flow and doesn't need enlargement, just clear.
BTW: For stock compression us a stock (head gasket). For 10:1 & above the copper one is better. The "main thing" is that both (block/head) are flat and clean.
Good luck.
