I don't know the width of the widest part of Series 2 / 2a bodywork but Rover used 52" track axles, the track you use depends upon what offset there is on the wheel rims you plan on fitting.
According to the ALRC regs a Series II/III is 64" wide body and 64.5" at the bulkhead. However it's worth noting a 90/110 (Defender) is exactly the same it just has wheel arch spats/flares to cater for the wider axles used on them.