its borderlione if you'll get away with it at that power, if you give it some mechanical sympathy you will possibly be ok.
There 3 options you have for upgrade route,
1. hawk 5 speed box, these are alot storng than the older 5 speed box's, i ran 420 ftlb for 14k miles with no issues.
2. Aftermarket gear kit, not mega cheap
3. 6 speed setup, again not cheap, but will be fine for what ever power you want to go. bear in mind you'll need all the running gear bits to suit, rear diff, drive shafts, hubs etc etc
Tbh i'd be looking at the 6 speed route myself. I have had all 3 options over the years and while 1 and 2 are strong, the longer ratios on the 6 speed suited the extra power better.