I think you'd be better off selling your Minor and buying one that already has a V8 fitted if you want a V8 Minor
A massive amount of fabrication is needed, including making a whole new seperate chassis and working out how to bolt the body to this. This requires a huge amount of engineering skill, welding, metal fab, coachbuilding skills knowledge of suspension set ups and making different parts from different cars work together. Plus, you will lose historic status, suffer the ignominy of your car having a "Q" plate, like a kit car, as it is a new conversion, you will have to pay tax on the car forever more,and probably at the higher band of tax (£200 plus), plus the £400 BIVA nightmare test to put the poor car through.
Sorry to say, but if you have to ask how to fit a Rover V8 to a Minor then I'm afraid you do not have the skills to do the job, so I would say do not attempt this as without already having acquired the skills and knowledge then, I'm afraid, you'll end up cutting up a perfectly good Minor and it will end up being scrapped.
From a recent thread on this board about "how many are left" we should be saving the cars we have!