My main concern is the body control. On good fastish roads I notice a slight floating corkscrewing sensation. It seems to me to be either springs or damper related.
My thoughts are that the shell is not very stiff, so would a strut brace sort it, if so which type would I need?
hard to know what the issue is from your description - such is the nature of describing it over forums.
but, it could be any number/combo of things:
*chassis-flex: a FSB (front strut brace) can help
*dampers: not a strong point in alfa's; upgrading will certainly improve ride quality and body-control
*springs: unlikely to be an issue; change for 'lower' if you want
*ARBs (antiroll bars): unlikely to be an issue; most people don't change these except for track enthusiasts
*tyre pressures + quality: ensure these are appropriate; 'cold' pressures should be around 36psi front, 33psi rear; are the tyres actually good quality (presumably so)
*wheel alignment: can only adjust toe (camber is not adjustable); factory settings are 'front toe-out' and 'rear toe-in', both of which are biased towards stability (when cornering) and reducing tramlining; for a street car, worth keeping the rear toe-in (to avoid lift-off oversteer), but can set front toe to neutral
bang for buck, i would pursue:
*dampers: KONI Sport, KONI FSD, Bilstein B6
the other more major upgrade that is well-worth considering is the Q2 differential. cos of the relative-cost, i'd put that 4th on the list, but it does make a big difference to overall power-down handling.
hopefully you sort it out to your liking.