He says the boys have been "level-headed" at training this week, keeping a lid on the emotions knowing that underdogs Massey will be throwing everything at them.
While the Inisi brothers - No 8 Lotu and second five Fine - are pivotal figures for Westlake, they also lean on lock Blake Houghton, props Finlay Brewis and Leon Thambiran, plus the halves Keepa Wiki, who will kick the goals, and captain and No 10 Jack Heighton.
"We did enough in the semifinal to build a bit of confidence," says McGahan. "Without being disrespectful, we thought we'd get through that game."
Massey's victory over Rosmini did not come as a big surprise to McGahan, as the west Aucklanders had been resting key or injured men in the last fortnight of the round-robin.
"Massey are a good, young side."
One would think any complacency Westlake may have harboured was jolted out of them with the knowledge of a rousing first spell by Massey in their first round-robin clash.
Westlake rallied to win 24-12 from a 12-0 deficit and took the return game 27-5, but Westlake's defensive systems will need to be on high alert to subdue the likes of Massey loose forwards Christian Lio-Willie, the skipper, and No 8 Nathan Va'atausili. Jackson Wilson comes in at lock to cover injury. Mac Pisi, the last of the Pisi clan, will suit up at fullback.
Massey have not always been consistent in 2016, but comfortably qualified third and pose threats across the park.
"Rosmini have a heavy forward pack and we had to build our game around defence," says Massey coach Brendan Curran. "We'll again need real enthusiastic defence tomorrow night and that's been our focal point in the last two weeks. But the boys are really excited."
He is not thinking about it, but victory for Massey would represent a notable double for Curran, who helped the school's director of rugby Jarrod Framhein guide Massey to the North Harbour premier club title last month. Those commitments saw Massey and North Harbour prop Adrian Smith and former Harbour assistant coach Alex O'Dowd, now with UK club Nottingham, help run the First XV until quite recently.
There is sure to be vocal support for both sides in the final, and the union will be hoping most of those fans stick around, unlike in recent years, for the Mitre 10 Cup opener which follows.
Regardless of tomorrow night's result, Westlake will, on August 27, play the winner of Saturday's Auckland 1A final to decide the Blues' representative at the Top 4 tournament in Palmerston North. Normally Westlake would have played Northland champions Whangarei BHS next Tuesday, but the latter passed on the game after losing their North Harbour semifinal, not wanting two bites at the cherry, as it were.
Massey, competing in the Blues Top 4 co-ed playoffs, will face Kerikeri next Tuesday to find who will play Aorere on August 27 for a place in the Top 4 tournament.
North Harbour 1A First XV final
Westlake BHS v Massey HS
QBE Stadium, Albany
Thursday, 5.15pm
Westlake BHS: Cameron Hey, Spencer McDowall, Baylee Katipa, Fine Inisi, Arikitoia Hood-Kaitapu, Jack Heighton (c), Keepa Wiki, Lotu Inisi, Jacob Katipa, Tupou Afungia, Blake Houghton, Zach Lassen, Harry Collard, Jordyn Evans, Finlay Brewis; Reserves: Luke Jones, Leon Thambiran, Harry Forbes, Arno Mouton, Casey Smith, Cooper Green, Jack Fuller
Massey: Mac Pisi, Dave Puleiala, Alex Tavita, Seb Tielu, Sam Samu, Nikau Drage, Miracle Naititi, Nathan Va'atausili, Jamie Casciaro, Christian Lio-Willie (c), Jackson Wilson, Sone Teo, Vili Sio, Christian Lemalu, Grecia Taele; Reserves: Tevita Langi, Nadi Tulia, Keenan Matthes, Filipo Talitau-Tio, Bill Halo/Blessed Tumua, Anthony Naititi, Ryan MacMillan