Limits of the standard R32 GTR gearbox?

[yamaha227]

After a session at the drag strip last night i had alot of problems changing from 2ng to 3rd under load on the strip. it crunched a few times and wouldnt go in to 3rd until i slowed right down.

On the last run it crunched again, then when i went to park it i had no reverse gear, it will go in but when you let the clutch out it just sits there. found out on the way home that i also have no 5th anymore aswell, so now i have to way up what to do.

Any ideas on what the standard gearbox is capable with handling. I intend to get the car into the low 11's, thats my goal anyway. Will prob only go to the strip a couple of times a year and the rest of the time will be driven on the street.

Current tune is about 320kw@rw but hoping to get somewhere around 400 to achieve my low 11 goal. Should i go for a new standard GTR box or something a little stronger??