Originally Posted by Peter3442

My comment related to how the tubes were joined together, which is what we were discussing, not their cross-section. Brazing and bronze welding were well established in the motorcycle world especially for thin wall tube. Though round rather than square, you will have seen that the Manx Featherbed was also Reynolds 531 tube.

Originally Posted by The Mekon

I think this project is very courageous! I have a 1972 V12 chassis - the engine has gone to be a work of art, and my original intention was to put a 4.2 engine in it. The body turned out to have more problems than i was willing to fix so the car will be scrapped eventually. I think you need to re-examine the weight figures, A V12 engine is heavier than an XK once you add the cast iron exhausts etc. The block is not light, and the crank is a monster!
The 3.31 LSD rear is bound for my running 1973 XJ6 once it gets its 4 speed o/d box. Will give me around 29mph/1000rpm in top o/d which the six should pull.
If you are really serious about this project then I may offer you the tacho and speedo from the V12. I have had lots of interest in these - they are rare.

Originally Posted by Peter3442

Given the effort required in the project, I'd keep the injection and consider either a manual shift or a more modern automatic transmission with 4 speeds and lock up.