I've got the same basic problem for my mk1 going V6! so thought I should look into it.
quoting the VOSA website:
Acceptable changes
It does not count as a ‘substantial change’ if:
1 changes are made to preserve a vehicle because the original type parts are no longer reasonably available
2 they are changes of a type which can be demonstrated to have been made when vehicles of the type were in production or within 10 years of the end of production
3 axles and running gear have been changed to improve efficiency, safety or environmental performance
4 changes were made to vehicles that were previously used as commercial vehicles, and you can prove the changes were made when the vehicle was used commercially
Yours would be number 2 as Neil says so youre good to go!
fitting a v6 in your case would hardly be number 3....
But in my case i'm fitting a 5 speed and fuel injection so I can argue number 3 as its bound to be cleaner and more efficient than a v4 AUTO
interestingly if people are struggling with these, all you appear to need to do is use it comercially -

say towing a lrage trailer with a v8 and then apply after doing that and you cover it as number 4!, but the other thinbg to watch out for is that once tax and MOT exempt you cant use it for business, not even popping out to a boiler braekdown say, unless you do it for free

which does mean I cant use mine for recovery if I'm going to charge for it.
Ollie