alfa internals from the 166, 156 and gtv are interchangeable with the coupe internals, you could swap the final drive ratios, if you could get a hold of a second hand 6 speed box you could put the internals or just the final drive from either the alfa gtv, 156 3l or 2.5l in to the box, the final drive from the gtv is 3.563, from the 3l its 3.733 and from the 2.5l its 3.93 in 6 speed boxes, you should be able to use these final drives in the 5 speed as well
http://locost7.info/gearcalc.php if you download the gearcalc from that you can put the different final drives in to see what effect it has on the speeds in gear, ratios for the 5 speed coupe box are
final drive 3.111
1st gear 3.800
2nd gear 2.235
3rd gear 1.520
4th gear 1.156
5th gear 0.914
6th gear 0.816 on the 6 speed boxes, these ratios are the same for all the alfa 6 speed boxes as well other than 5th which is 0.971 as oppose to 0.914 on the coupe, alfa altered the gearing on its 6 speed boxes by changed the final drive