T5 and T9 bellhousings are very different.

Considering that you are in the UK I'd go with a properly built T9. We run T9 boxes here in North America with turbo Sierras all day long. I think a Merkur XR4Ti weighs a lot more than Spitfire with crappy shock loading from an open diff. An uprated T9 in a spitfire will be more efficient and will have better gear selection that a T5.

I may be completely wrong though.