14" Coffin Spokes

https://www.imagewheels.co.uk/billet-81-alloy-wheel/

 

You'll likely have to go with the 15s - the original coffin-spokes for the Stratos and Michelotto 308s were 15 (replacing 14" Campy on the Stratos).
If you want real magnesiums: https://www.alfaholics.com/other-marques/lancia-stratos/lancia-stratosferrari-308-gp-4-magnesium-wheels/

Also: http://www.rointechnology.com/indexx.php?link=pagina_cerchio.php&nome=Lancia Stratos&foto=Lancia-Stratos.jpg&marca=lancia

 

Proportionally, the 1/2" reduction in sidewall height won't be that noticeable visually, or ride quality (and protection from sharp pothole edges). I know "that look" you're trying to retain - I had 18" Forgelines made for my Porsche that came stock with 19" (and 20 or 21 available from Porsche itself as options). I'd have gone with 17s, if there were decent tires for the sizes I wanted.

 

I wonder if you can fit the 15x8 fronts
https://www.alfaholics.com/other-marques/lancia-stratos/lancia-stratosferrari-308-gp-4-magnesium-wheels/
on all 4 corners - offsets for early 308 are same front and back (I have a early ‘77 steel gtb). Was the front fender of the 308 gp significantly widened? Would these fronts fit the stock front (and rear) fenders ?