You need to determine how much power you reasonably expect, and what you are going to do with the car. There are tables with widths of various Toyota rear ends, but there is no reason why you need to stick with Toyota. I think you will find references to skyline diffs as well, though I don't know if that's a good thing.
Shortening a diff is not as simple as you might think either. And it will need conversion to leaf springs, plus you will want a panhard rod. And you will need disc brakes. The car will clearly need engineering.
And you really should look for something with an LSD or ability to swap an LSD in.
As Altezzaclub mentions you also need to try to minimise unnecessary weight.
Is a T series 6.7 inch centre / axles going to cope with what you will throw at it? What width wheels/tyres are you planning to use. Will you be able to fit them under the car? Probably anything with a larger diff centre will be significantly wider than a Ke55. Needs to be factored into overall width. And a larger tyre footprint will place additional load on the centre & axles.
So it's complicated. If I was you, I would consider above, then go into a diff place and tell them what you want, and see what they can build for you off the shelf. Probably cheaper than sourcing a differential unit yourself and converting it. Then go back to your engineer and see what they are willing to sign off.