PS: back when I was doing the conversion, I was interested in keeping it within the HRCR Historic rally car regs; I don't know how proscriptive they are nowadays but in those days you could basically do anything to a new-build historic-class car, that had been done to them when they were 'modern'. So if you could find evidence of a previous car having been built in a particular way, you could do it.
In about 1973, one Ian "Del" Lines built and campaigned a 2-door Toledo Sprint rally car. Back in the late 1990s I went to see him at Weston-Super-Mare and he was a mine of information about the car he had and gave me (a.) lots of encouragement and (b.) a letter confirming some of the key mods that he had carried out on his car, which then gave me the historic provenance to build mine to the same spec. I'm guessing that letter is now lost along with - from the sound of it - much of the spares package I'd built up
As at 2005, Del Lines was still active in the Rover SD1 Club as the club's Technical Advisor, but carrying out his duties remotely from his home in Marbella - hopefully he is still going strong (he will be 75 this year).
This interests me greatly. As I am hoping to do the same shortly but wasnt sure how prohibitive the HRCR would be with changing engine. But I am sure the rules still allow for "as you said above" time relevant spec changes if car had them while rallying in that period.