George,
The correct OEM tire size is 155 / 80 - 15 (well, actually they were 155 / 78, but you wont find those
. If you prefer wider tires (along with wider rims), you need to pay attention to interference with the steering arm in front. I know that Centerline recommends 185 / 65 - 15 as a substitute size (same OD, but much larger footprint), but check the clearance with front suspension bits and the wheel well when the wheel is turned. I'm not too keen on the look of much wider street tires on the Giulias, especially one as nicely restored as yours. But to each his / her own.
And great work on the chassis upgrades! Re-inforcing the trailing arm support is an absolute necessity, and your approach is a good one. When I restored my Sprint, I had plates welded across the front and rear of the support so they would not collapse under hard cornering. By the way, some of the vintage race groups require this type reinforcement on Giulias just to prevent collapse of the support during a race (it HAS happened, and it's no fun).
Sorry I don't have any spare Fergats, but if you want to go to wider tires, you could consider the Panasport style rims (definitely available for the Giulias). As for the P3's, you were lucky to find a new set in good condition. I had to throw out a set of 13 yr old P3 tires today that had plenty of tread -- but also had cracking in the side walls. Oh, well, better safe than sorry.
And if you're looking for Carello turn signals, Re-Originals supposedly has them, as well as Classic Alfa parts in the UK. Certainly not cheaper than Centerline, but they might be real Carellos. It might be better to just keep tracking e-Bay and Alfa swap meets, because the originals are still out there.
Jim