I'm here! :wink:
A 1963-registered Herald 1200 likely has the later and stronger (pretty much mandatory for anything bigger than the 1147) chassis. So long as the commission number is GA80000 or later, or GB xxxx and beyond, you should be ok. Or, simply flip open the bonnet and see if a: the front crosstube is bowed out in the center and b: has two additional braces -- one outboard of each front main rail extension) -- supporting it. If so, you're good. Another obvious clue (although not 100% reliable) is where the engine mounts are. If they're on the chassis mail rails, it's early chassis. If they're on the suspension turrets, odds are it's later chassis.
No, that "G" engine would not be original to any Herald 1200 with any GA or GB commission number regardless.
Yes, the Spitfire 1500 engine will bolt in, but I would plan on bolting in the "matching" gearbox that I hope you have. If not, plan on getting hold of at least a 1971 or later all-synchro, three-rail gearbox. Plan also on upgrading to a 1971 or later Spitfire differential. Your propeller shaft can stay, but it will need new u-joint flanges to match the gearbox output and diff. input. Same with your rear axle shafts; they'll need the larger flanges as well.
Having gone that far, you should be ok with the original rear spring, even though a later diff. will have only four hold-down studs; I'm told that this IS sufficient. Or you can upgrade to a Spitfire-STYLE swing spring, but you really need to get a proper one for a Herald, which Canley Classics in the UK sells. Most everyone seems to agree that a stock Spitfire swing spring isn't quite enough for a Herald. (Oh, and you'll need the thicker later Spitfire front anti-roll bar, and I'd suggest upgrading to Mk3 Spitfire or later front disc brakes, even over the optional disc brakes your Herald might have.)
Good news is that this all pretty much bolts in, with probably the biggest obstacle being adapting the exhaust system. Start with an appropriate Spitfire head pipe and work your way back from there (perhaps "custom" exhaust route would be easiest and cheapest).