I stuck with the steering box (I just wanted to get driving again asap and didn't really consider a rack to be honest) - mine was leaking badly when I got it and after a longish search and thanks to help from others on here who suggested them, I sent it away to Kiley Clinton who despite the name of the web site do tackle steering boxes
https://steering-racks.co.uk/
What they did appears excellent so far, the steering is very good. I did go right through the entire suspension/bushes etc etc as well.
The rack and pinion would be a great upgrade, and future proof a vital element of the car. Look forward to hearing how you get on - it looks like an awful lot of work!