The only Torana I've found so far with any original specs is the L34. It had 2" engine pipes and 1.62" tailpipes. I think this was the same as normal L31 SL/R as well as exhaust past the headers was free under Group C.

I also just looked up the HQ Service manual volume 4. It states as I thought all along:

Engine pipes - L6 1.875" OD, V8 2" OD. Tube thickness 0.06".
Tailpipes:

L6 1.625" OD. V8 engines (except GTS350 or N10 dual exhaust (253 and 308)) 1.75".
GTS350 or N10 dual exhaust on 253 or 308 - 2".

As far as i'm aware HJ-HZ was identical (except no 350).

Note this is passenger vehicles. The HQ commercial vehicles supplement issued in late 1971 shows the exhaust differences (mounting) but makes no mention of any size in pipe changes (it would if they were different).