When is a 904 too small?

At what power level is a 904 no longer big enough?
Here's the story.
a couple of years Back My dad bought a drag free 904 w/full manual valve body from a reputable shop. The car is an Aussie Valiant hardtop. (Very similar to a 68 Dart) Runs a 340/418 stroker.
In the time it's been in there, the car has done around 1200 miles. and about 20 quarter mile passes. (best to date of 12.5@107 running a 2.92 rear end, and no rev counter yet)

thing it's doing is, under full power acceleration there's a flare on the 2-3 shift. So I messaged the shop who built/supplied it.
their reply was yeah, I'd say the High gear clutch is worn.
But after so little use??!! Then they said how much power you making? I dont know the actual numbers. but I told him what Times I've run.
Then they say yeah the 904 is limited in what high gear clutches it can take. And theyre considered a lighter duty transmission.

So are we dreaming to run a 904 in this car, or do you think he's downplaying that the trans they built isn't up to it?
20 passes, and 1200 miles.

What do you guys reckon?