is it feasable that a standard diff will handle an a15 with side draughts?

Depends how many donuts you do at the local mcdonalds carpark.

I would think it should be ok, but a Ute H165 would be a better choice.

A lot of 30 year old diffs have gear wear and bearing wear, and when put under load, the gears will more easily 'ride up' on each other, and self destruct!

If your H145 is in good condition it will hold up. We have members with 150hp A-series running H145s for many years without problems.

I agree an H145 could last behind a 150hp A-series.

But beware: for high-torque A-series, I doubt it will last. Remember, it is torque that breaks gearboxes, not HP. My stock A15 broke two H145s that were in good condition, with just regular spirited driving, no high-rpm clutch dumps, donuts or other abusive behavior.

Tom,
If this is your ute you're talking about, It will be good as gold. The 1200 utes have an H165. (165mm crown wheel)
I don't think you'll have any reliability issues.

It's not so much a matter of whether it will last behind the engine & more of a matter of whether it will last behind the driver.

If the differential is in good serviceable condition, then, with the right driving technique, it will be fine. Keep the shock loads LOW & feeeeeed it some throttle rather than dump full power onto it, then it will serve you well.

will the H165 stand up to a stock SR20 vvti engine?
I know that they are pretty strong. and the few drives I have had in the ute it seems like its not a weak point but I will only find out once I can do a bit of spirted driving.