Substitution for beef (veal) in a stew
I'd suggest skinless bone-in chicken thighs, as they have plenty of fat and collagen to keep them moist and tasty. I've cooked them in French-style wine-based stews, not to mention cacciatorre, for 2-3 hours before now and they just fall off the bone. 

It is virtually impossible to overcook them, unless you boil them mercilessly for hours. Just get a nice gentle simmer going - not only will this make the meat tender, but it will improve the flavour of the tomato sauce as well.

Do not use chicken breast - it is far too lean.