Because of the differences in the chassis design I'm not sure that a Spitfire header would work, but there are high quality race and street headers available for them. You could check with Rimmer Bros.
http://www.rimmerbros.co.uk/rimmer/triumph/spitfire/exhaust#Sports%20Exhaust%20Manifolds
'73 Midget (V6)
'59 MGA (I6) under construction
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