8" wheels should fit, so long as the offset is correct. but i don't know anyone here could guarantee it, as it may depend on your exact setup (coilover size, how low you wind the rideheight down, etc).
as such, you probably don't want to risk buying online. buying from a retailer with a storefront means they could try it out and see; if it doesn't fit, you don't buy them, as simple as that. much less risk and hassle this way. though you probably already realise this!
i had 18x7.5" wheels on my TS 156, and there were no probs. appeared to be room to spare. KONI's + Eibach springs, not coilovers (though coilovers should provide some extra clearance on the 'inside', again dependent on camber).
for offset, i would recommend +35 to +38mm. much less, and you risk the outside of the tyre rubbing the wheel arch; much more, and it risks rubbing the coilover.
note, i have 8.5" wide wheels on my GT without any probs. i would expect the 156 to have similar clearances, though i'm not 100% sure.