[DaveW]

HI Shelley,
I have a lot of experience with Toyota 5 speeds.
As dhs.buckley has said, the alloy case Celica box (T50) will not last long with any abuse, very small lay shaft and gears.
With the Steel cases, there are 2, W50 and P50, of these the P50 can handle mammoth torque, it has huge shafts, Gearset and syncros, but the bearings are fairly low rated, fortunately, they are all standard spec bearing sizes and can easily be swapped for much higher rated options in the same size, I regularly swapped out for TSK, SKF products then fitted to modified 302's 308's 351's etc with reasonable HP.
The P50 box was also used in the Dyna trucks, hence the upgraded load ratings.
It is also the exact same box as the Dyna P40, but with 5th gear.

However, if you are going to swap boxes to get to this reliability, you might as well swap to a box designed from the outset for motor power you have.
Although along with this you will also have to consider clutch slave cylinder or cable pull brackets etc as these will probably have come from Hadfields, Dellow or some other to suit your current mix and match.

cheers
Dave