I'm a little confused. If you got a Turbo 2.5L why would you sell the Turbo car to turbo your S2. And I have know idea why you would want the 2.7 head. (That's a 944S non-turbo right?)
Turbo Valves are Sodium filled, and I think the Turbo has bigger exhaust while your S2 has Bigger Intake. Not sure on that thoe. Also the Turbo Head is specially designed to handle the extra heat.
If your goal is a 3.0L Turbo You'd start with a 3.0 L short block, and insert 104mm 8.5 CR Piston. Would require aftermarket Pauter or Carrillo Connecting rods to handle the Power. Machine work to notch the upper Crank Cradle for Conn Rod bolt clearance. I would think some custom exhaust for Cross over and waste gate. Stronger clutch and flywheel, Electronic management like a tech3 or Motec.
If I had 10K$ to drop in to a project like that I'd go 3.0 short block into a 944Turbo/S Car, use the rebuilt Turbo 8V head, Racing Springs, Un-obtainum Retainers, and Re-worked CAM.
This guys site is a must for you cause that's what he's doing.
http://www.fastest944turbo.com/
I think he's using
http://www.944enhancement.com/html/engine_building.html which explains the engine building very well.
The All mighty Clark of
http://www.clarks-garage.com/ under engine modification has some great stuff on all of this as well. (cause he's done it)
If you got Say 20K$ you could Turbo A 968
http://www.powerhaus.com/944t.htm
So I'd sell the 2.7 head to Ae1969. Sell the whole s2 Car. Sell your Turbo Short Block, Buy a 3.0 Short Block and S2 Stage III+ Clutch assembly and Bell housing. Use the 944turbo as the super car. I'd think something like 3.5 to 4" exhaust but might be able to use a piggy back Fuel air controller (not have to replace Turbo wiring) but then if you drop that much in to a project why short your self.
Did we mention it's really expen$ive.
Many ask such but few would dare.