You should be OK cutting and resplining HQ cab-chassis axles as they are fully heat treated like a 9" drum brake axle. I know you can't cut down HQ-WB banjo or standard salisbury axles as they are too soft once cut and splined - I know that from experience! Not sure if anyone will do them today though. Also, not sure how deep the heat treatment is on the cab-chassis axles, you have to cut a fair way in for banjo splines. You might be better off sourcing a 28 spline banjo centre and cut the cab-chassis axles to 28 spline - as you have shortened the diff you might be OK. I remember looking at shortening cab-chassis axles years ago for A9X length but you can't do it because of the way they are machined. But with your housing shorter you might be OK. I know the cab-chassis axles were OK to shorten to LJ length.
Are you keeping the 4.75" PCD?
Try ringing one of the Sydney region diff shops and see if they can do you billet axles in 23 spline. Might be cheaper and stronger in the end. These guys list them, but they are in QLD I think:
A9 FLANGED CUSTOM ALLOY RACE AXLES 23, 25, 28 & 31 SPLINE - Altra 9
I've tried a few in sydney and melb but they are only interested in selling 28 spline billets to match a trutrac centre.
I'll give that QLD mob a go and see what they say.
It was meant to be a cheap experiment as my only other options was to change rear wheels or buy another diff lol
Thanks for your help though
J