Stick with the Thor 4.6.
I've only see one successful conversion to a LS 6.0, on the 'other' forum. I know that Overfinch used GM engines in their P38 conversions, but there weren't that many of them done, so doubt that the parts/adapters/electronic workarounds they used aren't readily available anymore.
The one done in the thread on rr.net started out with a hiss and a roar, and I think he was planning on making kits to do the conversion, but I also know there has been a lot of teething problems between the engine/box and having to adapt a lot of linkages, wiring looms and all that - so going from that, and the fact he works on engines a lot and has all the tools to be able to fabricate/adapt parts, then I think as far as a DIY project, it's probably a fairly involved swap to do - especially on this side of the pond, as the LS engines are commonplace in the states, but not so much over here...
My thought process is more along the lines of if I want more power out of it, then I'd be looking at going to someone like V8 developments and get a 5.2L short Rover engine and then build that up into a replacement Thor V8 - so you know that everything will mate up and run. Tornado Systems (Mark Adams) who did all the bits for chipping the GEMS engines now does the Motronic systems too, so if I went down the 5.2L option, then I'd look at getting the map tweaked to make the most of it.
Would probably work out quicker and more cost effective in this country go down that route.
Not saying it's impossible, though! someone has to be a pioneer in the UK...
'06 4.2 Supercharged Black/Cream "Jess" -- Bought April 2021 (NZ)
'01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
'99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
'95 4.6 HSE Epsom Green/Tan -- Traded June 2008