since I last owned a Ke70 20 years ago.

My question is I have chosen a ca18det engine for my conversion and looking at help into what rear end I should be looking at as I'm sure the standard wouldn't cope.

Is there a straight bolt in diff axles and drums I can put in for a bit more more strength?would a carona diff fit and do the job?or and recommendations would be great...

I will be putting a big brake upgrade on the front.

There's a range of diffs you can fit, although the leaf-spring ones will need 5-link mounts welded on. I expect all of them will need a new driveshaft-diff end.

The 5-linked Celica T-series diffs bolt straight in, that's what I use.

There are F and G Toyota diffs off heavier cars, they will need mounts, and really, anything out of a Hilux or van that is designed for carrying more weight.  Nissan Skyline diffs too.  Along the way you will find them getting too wide, it depends on your car use, so they need to be shortened, which makes the conversion quite expensive. More modern vans use 5-links and come with disc brakes, I've run a tape over a couple of those.

You'll just have to spend a few hours at the wreckers with a tape and a camera..