I think you will find that it is MG Midgets and Bs and Cs that have a monocoque shell and your dads Spitfire (along with the Herald,Vitesse,GT6 and the TR 2-6 series) definitely has a chassis which the engine and suspension and body attach to and can clearly be seen in some of the first photos you showed painted red before the body was refitted. IIAC only the 1300/1500/Dolomite and the 2000/2500 saloons, the TR7 and the Stag are monocoques:eyebrows:
Lovely car BTW Triumphs always look good in red and as Chris said once the rear suspension settles will look even better:D