Hi align600,
Nice choice of coil!
That's a fairly easy one. As far as the lower shafts on the Minelabs go (assuming that's what you're running) they are made of fibreglass. Just checked my ones, both the one that came with the 5000 and the spare I have on my 16" NF Mono.
Not so sure about changing the upper shaft though? I have fairly minimal experience on the Minelab with a big coil. Perhaps someone with good time on a large size coil like the 18" will offer the advice needed.
Kindest regards,
Shauno.