Brackets on my chassis are universal for 7 or 7 1/2" diffs so the standard bolts are too short.
Present diff has helicoils and short bolts from each side.

New diff doesn't have helicoils and I have no way of putting them in.

I have one long bolt from engine mount on a scooter engine and 3 metres of threaded bar.

Is the threaded bar OK for the diff mount or am I missing some facts that require a high strength bolt? I have no idea what the rating of the threaded bar is.

My MAC#1 kit included the threaded rods for diff mounting, so it is an "approved" method of mounting.

If you're not sure of the bar rating, that could be a problem. Maybe try hacksawing 0.5 cm from one end, if it's like butter then get some rated bar from your local fastener supplier or trade nut and bolt shop.

I asked before about threaded stud and the general opinion was it was not up to the job.

Might be best to get the new diff tapped and helicoiled - engineering firm would do it in half an hour easy.

My STM kit came with a proper bar with a thread at each end. No chaffing

Studing is fine so long as it's HT this is difficult to prove unless you bought it in the first place!! Stuff I have has an 8 stamped at each end (not much good once you've choped it off, might be worth checking you studding).

Dan