Mat, The problem with Alfa transaxle syncros stems from high rotational mass and wide ratios. The syncros function is to bring the set of gears that are about to be selected to a speed that enables the driven gear to lock to the output shaft. As all gears ( except reverse ) are engaged all the time ( driving and driven ) the clutch plate, input shaft, 1st 2nd 3rd 4th and 5th gear sets all have to change their rotational speed to allow the next gear to engage. Slower to change up and faster to change down. The closer the ratio, the less the speed has to change. So, to fix the syncro problem the mass of all the aforementioned bit needs to be lightened, or the ratios need to be closed up, or you change real slow and double clutch on the change down.
To lighten, a button clutch is the easiest, cheapest way to get rotational mass down. Anyone with a set of spanners and a jack can achive that in a few hrs in the garage. I haven't put one of those in the GTV6 yet ( but it's on the list). Then if you know someone handy with a lathe and ceramic tips you can remove a fair bit of meat from the center of the gears. Pop the syncro carriers off, do both sides down to the soft stuff in the middle, drill holes all the way round ( probably only possible on 1st 2nd and 3rd because the gear diameters start to get a bit small) and you will have much happier syncros.
To close up the ratios is a bit more involved. Most efficient way is to simply put twin spark 1st and 2nd in a box with Alfetta 3rd 4th and 5th. 3rd and 4th are an interference fit on the input shaft. press them off ( needs about 8 tons ), 5th is splined on with reverse. you need to be carefull with the choice of 5th. the twinny from memory is a .78:1, while the Alfetta is .83:1. most Alfetta 5ths wont fit on the twinny input shaft as the spline position is different. There are some boxes around that have a .78 5th that will fit the twinny shaft. Also there are a couple of different reverses. the three reverse gears need to be used as sets. mix them up and you will have a real surpise when you go to grab reverse!
My Alfetta rally car had a twinny 1&2 alfetta 345 and a 4.3:1 final drive and 195/65/15 tyres. Wouldn't have called it a fire breathing monster but it was no slouch either.
I have played with the box a bit for the GTV6, now has Twinny 1st 2.875:1 and 2nd 1.72:1, Alfetta 3rd 1.35:1 all lightened, a special 4th 1.13:1, and an Alfa 90 4th as 5th .946:1 still with the 4.3:1 and 195/65/15 tyres. When i do the engine and tail shaft change, i'll pop in a button clutch.
Anyway, you have travelled in it, works like a charm don't you think?