Personally and most people know my opinion on modifications to a 1960s Jaguar I would go down the route of the 420 front subframe and steering. The 420 as in Glyns picture comes with the Adwest Marles Variomatic PAS which is far superior to the older Burman system. It is period to the car, it will have the cut in the front subframe for the PAS to sit, does not need any modifications that will affect steering geometry, has less lock to lock turning (2.5 turns lock to lock instead of 3.75 on the Burman) is light and still gives feeling through the steering wheel. Lastly you can buy (as I have) a Dynalite alternator which allows the original PAS pump from the S Type and later 240/340 Jaguars to be used. The 420 front end also has a brake upgrade.
The cost of buying the parts is relatively cheap compared to converting the car to rack and pinion system and the labour involved in fitting it or buying a new electric PAS system and again having it fitted. The 420/240/340 front ends with the Adwest are a straight swap involving no more than half a dozen bolts. The down side is finding the parts but it would still be easier than finding all the parts to cobble together a rack and pinion system. One of the upsides ( there are many obvious ones I will not go into) is the cost can be off set by selling on your original front end after the swap. 420 front end might cost $500 but you could get back $300 for your front end. This is one for sale on E Bay in the UK. https://www.ebay.co.uk/itm/155640647...Bk9SR7yb9PbGYw
Search for someone breaking a 420 (not the 420G as this has a different steering box) for the better brakes or for a 1967 to1969 240/340 which will come with the PAS pump on the back of the dynamo and Variomatic PAS.

 

If I remember correctly, I paid £530 for mine. It looks better than the one presently on eBay. The subframe is rust free. It came with new brake cylinders and discs and suspension bushes. And the Varamatic box seems to be free of leaks, though I expect I'll rebuild it anyway. The only thing I can say against it is that lifting it out of the back of my X type wasn't good for my spine.

As for relative cost, an XJS rack and a manufacture of a pair of brackets to my design came to £45, but that was in 1990. After that I have up keeping a record of costs.

 

What I show is from an S Type & 340. The 420 used a combined Saginaw pump/reservoir running at 1200psi whereas the S Type & 340 used a pump at the rear of the generator operating at 1000psi and stand alone reservoir as pictured above.