I guess a relevant question is 24 valve V6 or 12 valve?
Both have been done into a Corse "S" chassis but with the 24 valve I'm sure you would need to alter the rear frame of the cradle. I have some pictures which may help in either case.
You should be able to source IVA compliant side windows from Plastics4Performance last known in Ashton, Gtr Manchester.
Look em up online.
If you are more confident getting the car through IVA with the current motor then go for it, but be aware of others advice as you may end up having the car off the road for a long time after IVA carrying out the V6 conversion. Maybe a long long time!😌
Where were you lucky enough to find this?