Well yes Bigted it does all point to the fact it could have started life off as a VC-Mustang Mark I 1973-74,with G3 carby.Not only from what you have found but it also has the black hub caps which were only found on the Mark I.I am curios as even the base looks in fantastic condition,so why on earth would the engine have been changed?It looks like it hasn't had much use at all.Your making me jealous as I could certainly give that base a home with the correct engine on it.It would have looked more like this one.
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A good find you lucky bugger.

Here for a good time,not a long time.