Jez, better off buying a whole car running with a V8 in it. Swap all the 8 gear over to your car and stick all your 6 gear in the other and resell it. Not only does that ensure that you have all the bits to do the conversion but it should work out a lot cheaper.

Buying a motor may seem cheap but then there's all the other stuff you'll need along the way which can really add up.