I'm sure I remember reading that Panzer IV tracks could be made to fit but I can't find the source at the moment. I think you need to score the 'bar' at the wide end of each link to match the look. PM me if you need more and I'll have a hunt around.
In 48th scale maybe Tim can sell you a set of sprockets from his beautiful Comet kit (and I'd be interested too).
Found my notes and it's the later solid horned Panzer IV type track as in the Tamiya kit that you need see picture here
Missed Q2 Yes, the Firefly and Challenger used the same 17pdr gun but watch the muzzle break they did change, the Comet had a shorter version. Contempary photos are your friends here.
Be aware that the Overloon example was 'restored' in the 60/70s using Chieftain bins, Centurion/Chieftain roadwheels and the Cromwell track was used as it was available. I think a Cromwell sprocket was added as well.
Sorry for the edits: Copy and paste /brain interface issues