if you have enough room to fit a 6-speed c514, one from a mk1 punto 6speed has a very low gearing (1:4.9 final reduction). Everything bolts up the same, the box is just longer and it is only a question of having enough room for it.
Lancia Y (840) 6-speed has the same final, 1st and 2nd gear ratio as the 6-speed Mk1 punto but the upper gears are more "spread out". Still a good match for your needs.
The Mk2 punto sportings 6-speed have a more "civilized" 1:4.071 reduction, so you don't want that.
Your gearbox has either 1:4.071 final if it came from a sporting or 1:3.733 if it came from an elx or sx punto. No 5 speed box (that I know of) has a gearing as low as the 6-speed from a mk1 punto.
Final drive can't be easily changed as the pinion gear is one piece with the secondary shaft. You could disassemble boxes completely and transfer shafts (and use correspoding diffs) but that's a lot of work and you still can't make a very short 5-speed box as the 1:4.9 final was only made for 6-speed boxes (longer shaft). Bacci Romano makes secondary shafts and diff crown gears with custom ratios (very low, up to 1:6) and sells those sets for around 1100 eur. I think both 5 and 6 speed versions are available.