Enlarging bulkhead hole

[sphelps]

It can be done by drilling another hole around it but unlikely anyone would attempt it. Better just to work with what you have, I'd use the two 1/2" lines for the primary and secondary and the 1" as the back up. I'd also up the line size before and after the bulkhead to 1" like you mentioned, 1/2" bulkhead with 1" line will flow a decent amount, much more than 1/2" line throughout and not much less than a full 1".