The 1400 GT uses stronger rods and crank compared to the non turbo version.
The 1600 is not available as a turbo so the parts are not available unless you go to engine tuning specialists with your lottery winnings.
The 1400 and 1600 share the same bore but the 1600 runs a longer stroke. This gives a little more low speed torque at the expense of revs (where the power is) and having a longer throw, the crank is relatively weaker. Change the crank and you have a 1600 but it wont last long as its not built to handle the power.
The Lancia Delta HF 4x4 ran a 1600 turbo engine but you will be lucky to find one of those on eBay.