He'd better be, since he'll have a lot of bodywork to do. Basically, he'll either have to lengthen the Spitfire chassis by 7.5" between the front and rear wheels, AND add all the outriggers to support the body, or ADD all the outriggers to support the body AND shorten the Herald body by 7.5" between front and rear wheels.
So long as the Herald is approximately June-July 1962-built or later, which translates to a commission number of either GA80001 or later, or GB1 or later, all he needs to do is unbolt the Spitfire's engine, gearbox, and front and rear suspension complete* and bolt them up to the Herald chassis. After that, all he probably needs to do is to take the u-joint flanges off the Spitfire propeller shaft and put them on the Herald driveshaft, figure out a few minor connections such as the accelerator pedal linkage and front brake disc hydraulic pipes (to do it properly, they should come from the Spitfire).
[*He would need to keep the Herald's front springs. He could either keep the Herald's rear transverse leaf spring, or he could get a heavy-duty rear swing-spring from sources such as Canley Classics in England.]
There might be a few other little things to do, and I'm not 100% sure that he'd be able to use the longer rear axles (although I think it will work), but that's really the (only) way to go.
Oh, and if the Herald is earlier than the above numbers, it's really NOT advisable to make the swap. Period.