Steve, I think you're correct to consider the Caddy torque. The 8" can take more abuse than most think, but if you're going to play this car all out with slicks and occasional burn outs the 9" would be a much safer bet. I don't know how custom axle suppliers work for you folks across the pond. Here's a suggestion to consider. Yukon, probably others, make a cut to fit axle. Around here they're less expensive than the custom cut, plus you control the fitment. Basically they're an axle with a longer spline cut so there's room to trim:
http://www.yukongear.com/ProductDeta...px?ProdID=2174 This is only one of several options they have, search the site if it's something you're interested in.
Whether this pencils out with transport, duty, and whatever else you have to deal with I don't know.