Good to hear that you ask for stopping power. An upgrade is not necessary and in fact not possible, even counterproductive as it may mask problems.. A Fulvia (and Flavia) brakes better than most modern cars when they are 100% in order. All that is necessary to bring all parts within original specifications.
The braking force acts on the contact patch between tire and road. Visualize all the forces and counter forces going through the car. There is hardly a bolt or rubber or spring or ball joint which is not involved. If we have to do a brake job money no object we start with a complete print out of the wheel geometry. Things like caster and camber are not adjustable. That means the construction of the car has to be within specification to make sure caster and camber are OK and equal left and right. Many Fulvia coupe’s are warped.
Drop the subframe. Blast all the parts. Inspect for cracks and weak spots. Paint with the thinnest possible paint. Assemble with all new rubbers and silent blocs and use the proper tools to make sure it is assembled exactly to specifications. Disassemble spring, inspect, repair if necessary and assemble. Similar job to rear suspension.
The calipers themselves are the least important. There are few things simpler than a piston in a cylinder which is pushed forward by hydraulic fluid. Series 1 Dunlop or series 2 Girling does not really matter. The surface area of the pistons is important and it is equal on both systems. Just make sure they are like new.
Very important is the wobbling of the disc. If it wobbles it is alternating between the two pads rather than acting on the two pads at the same time. You have half the stopping power. The book specifies a maximum wobbling of 150 micron. We go for a wobbling of 30 micron if top stopping power is required. Discs have to be skimmed or new and within specifications. Make sure the brake limiter is OK. Best mount a new one and test the spring action of the arm.
Shock absorbers. Pads. And a few more things you will find on the way. I am now sitting in front of a computer, not a car. Ah, of course steering and suspension ball joints. Steering idler. Steering box. The cardanic joints in the steering shaft.
I look on a Flavia or a Fulvia as a mechanical computer. The prototype of the car was the Cemsa built in 1947 by professor Antonio Fessia. WW II was the pinnacle of the development of what I call mechanical computers like fire control systems and bomb aiming systems. The weight at the front of the Flavia and Fulvia is on the upper wishbone. That is unstable. Most cars have it at the lower wishbone. It is like the difference between an agile low wing Piper and a stable high wing Cessna. There are also animals like homo sapiens which have the weight on top of the pelvis. It is highly unstable. A lot of (distributed) computing power is required. The two aluminium towers with suspension, the three cross members, gearbox, engine are the central processing unit. The car is placed on top of that. Make sure all steel and spot welds of the body of the car are OK. The large bottom part of the subframe is there to keep the CPU upright.
With the wheels which are now on the car it is not possible to achieve stable braking performance.
See
https://en.wikipedia.org/wiki/Scrub_radius
With the standard wheels the scrub radius is negative enhancing stability. With the wider rims the scrub radius become positive. The car becomes sensitive to and unstable by left and right differences. And use tires that can be run at 1,7 bar (24 psi) so you have maximum rubber footprint. I like the Michelin 165HR14 XAS.
