hawk wrx box is designed for 2.5 so longer ratios. i ran one at 390bhp 420 ftlb for about 14k miles no issue on a forged 2.5 hawk.
options are,
A - hawk 5 speed, at that power at the top of its limit (assuming 450) and no telling how long it will last
B - Have your box rebuilt with a gear kit, but need to be carefull what gear kit you use as they have different ratings, also allows ratio changes and can be custom set for what youw ant (short ratio 1-4, lonnger motorway gear 5th), wont get much if any change from £2.5-3k. I have straight cut ppg gearkit in my old car and sounded epic haha
C - 6 speed from an sti, which is by far best option simialr price to rebuild with gear kit, downside is its 90kg heavier setup
had to decided on this option several times and tried em all haha,
One thing i did note was you say your engine isnt forged, if not forget anythign about 350 unless your planning on rebuilding again.