Curiosity got the better of me! And I got some possibly
useful information straight away from ePer.
The 1400cc 145 TS has a 5th gear ratio of 37/34 which would be 1:0.919 ( a bit taller than the standard GTV box ). But would it work/fit/be strong enough?
One element of doubt can be satisfied though : the gearbox casing ( part number 46807593 ) and surrounding parts are the same for the GTV TS as the 1.4 TS 145.
It's not too difficult using ePer for this. You're looking at group 212 ( transmission ) and subgroup 21215 in each case. Look at the 3rd and 4th drawing for each type and look for gears labeled 5a in the drawing.
Part numbers for the 1.4TS 145 are 7757949 and 7757950. Cross referencing 7757950 and you get a list of cars it was fitted to ( Tipo, Bravo, Brava, Marea, Palio, Siena, Coupe, 145, 146 & Fiorino ). However, and this is where it gets a bit confusing, cross referencing 7757949 gets a much longer list of cars which includes 156, 147, Doblo and !!! Spider GTV 1998-2006!!! . Yet the workshop manual as far as I can see only changes the 5th gear ratio for JTS engined cars ( 1:0.919 )
Needs more digging I think!