By way of an update...
I still have the nine hundred pound car, and have put about 3000 miles on it in the last few months. In terms up upgrades, it now has an Alfastumper undertray, a Wizard cat back sysatem, and there is a Quaife sitting on the garage shelf waiting to go in. The Christmas holiday was entirely consumed with the 156, which has had a full Eibach upgrade, Quaife and new clutch: that was a pig of a job, and I need a few weeks before looking under an Alfa again. However, Quaife without doing the clutch is pretty simple, and can be allegedly done without dropping the subframe, so should be a quick job on the GT.
Driven gently, it is getting 340 miles on a tank, but the temptation to spank it hard is very severe, and my worst so far is 240 on a tank. It is running Nokian winter tyres, and I have sourced another set of Jetfins for the summer tyres next year.
Apart from all that it seems to be a good 'un. I am chasing down little niggly squeaks and rattles, and have learned to live with the wind noise from the windows.
V6 manual is a copy of ePer that floats around on eBay - apart from the torque settings, it is not a lot of use. There are threads on here with download links occasionally.