why would you want to
Differential and syncros in the 1.1 boxes are shit from my past expereience.
IIRC the 5th gear ratios are the same on a VTR box as a 1.1 gearbox anyway.
But if i am wrong and the VTR gearbox does use a different ratio 5th gear the car would be revving its tits off in 4th and then the rev drop will be too much when hitting 5th.
But if the ratios are the same, then its completely pointless changing 5th gear.
you'd also have to think about what final drive you are using when going for this.
I am pretty sure the 1.1 and VTR boxes use the same gear ratios just the final drive is different.
Sure someone who knows a bit more can provide more accurate information.