ATM i've got a 4.11:1 in there as a legacy of my donor originally being a Herald 13/60 that was converted to a Vitesse.
After a lot of reading and even more time talking to Triumph spares places and dismantlers, i ended up going for a 3.63:1 diff for the time being as everyone kept telling me that the 3.27:1 diffs are "fragile".
I'm not doing any changes until after this weekend as i know that it's all working atm and i want to take it to the Bristol Classic Car Show, then my thinking is that I'll clean and recondition the 3.63 and put that in and see what (if any difference) it makes.
Depending on how that goes, i may still pick up a 3.27 to have a play with, but knowing that i can drop back to the 3.63 if anything does goes bang.
Here's an interesting link if you're thinking of having a tinker with diff/gearbox/wheel size:
http://www.mintylamb.co.uk/gearspeed/
It'll also be useful for those of us who simply change their wheel size!