Outer Limits 4x4 Board

I've got a 1978 FJ40 good condition and want to get 5sp and power steering. i have an opportunity to buy an unlicensed 1983 shorty with the specs I want but has some serious rust problems.

I'm not sure whether the rust can be fixed so that it will pass rego. how hard/expensive to fix rust in areas like firewall and around body mounts ?

I also have the option to put the good body onto the '83, or to take the good bits off the 83 - gearbox, front diff/brakes and powersteering - and put onto the 78 model.
any advice or experiences to share on any of the options would be appreciated.

A friend rolled his shorty and put a new body on the old chassis. I think it was pretty straightforward and may be easier than swapping all the other stuff over. I'm seeing him later today, I'll ask him.

I think the cross members etc are different on later models with 5speeds.

await the wisdom of DumbDunce!