Hi, my advice would be do not cut the Spitfire chassis.
If I'm reading this right you have 2 chooses.
(A) get a Herald chassis.
(B) cut down the original Miglia body work and cage frame to suit the Spitfire.
You have to bear-in-mind the Spitfire and Herald chassis are by their very nature designed to "flex evenly".
By welding a join in it you are creating a "stiff" area which will eventually cause cracks around the join. I know this because I have done it!!!
Option (A), most of the Spitfire bits will fit a Herald chassis and those that don't can be found cheap on E-Bay.
The Miglia cage frame, body, gear stick ect are designed for the Herald chassis so it makes sense to use the correct chassis.
Your build will be quicker, easier, less stressful and maybe enjoyable.
Option (B), If you welding and engineer skills are good enough to cut-and-shut a chassis then the cage frame would not be a problem.
As for the body work just look at some of the threads on this forum to find out how. While doing this you can also sort out some of the alignment issues.
Finaly as a bonus you will have a unique car that you yourself have created out of the original.
Only my opinion but above all, have fun and enjoy building it.
