Thank you to all for the advice
I'm pretty settled on a T5 with Quaife semi-helical internals (
http://www.gearboxman.co.uk/infot5.html). It uses the original Sierra Cosworth ratios (2.9 1st, 1.9 2nd ....)which will, as Peter Mulberry pointed out, lead to a drop in revs of less than 2000 when changing at 6000. That looks quite OK to me.
I have heard that this Quaife box has been known to break in Class A cars, but the grip at the rear on a typical road means you can't put more than 400 ft lbs or so through the propshaft, so I'm going to try it.
We'll see.
