12g940 head is the casting that basicaly in 98% of largebore (1275 style) engines, Avonbar are very knowegable on turbo stuff so would be able to advise on heads ( which they also do) or you can find out the piston dish etc and get a head at a better price from somone like Turbophil,
generally normaly non turbo performance heads wil have a 21cc head chamber, you'll probably want sonthing a bit larger to get the compression down to allow the boost to be crammed in... 28cc is perfectly feasible for chamber size, but you dont want to go too big otherwise you lose efficiency.
thr GT17 from the Saab95 is a good choice, the t25 is a bit old hat now, also for manifolds if you want to take the easy route talf to Matt at fusion fabs. http://www.fusionfabs.co.uk/