As I said: It is likely to struggle with 34, although fitting a Goat Link mech dropper would help. Campag only designed their road mechs to handle 26T max.
It will probably wrap the chain OK (so have enough room to take up the slack), but isn't designed for large tooth rears.
It is better to swap chainset, having a 42T small chainring is making life unnecessarily difficult.
There are no mix and match combos for a Campag 8 speed shifter, so you are stuck with a pre-2001 mech. If you can find a Campag MTB rear mech you stand a better chance with big sprockets at the back. For full shifter / rear mech compatibility details see here:
The rule is: don't mix one make of shifter with another make of mech. But sometimes you need to bend the rules, especially when you want really low gears on a drop-handlebar touring bike.