A good place to 'grab' 12V at the back of the car is the B+ connector on the alternator, which is connected via heavy cables back to the battery (via the starter). Just remember to use an inline fuse with whatever you connect there, as there ain't none twixt there and the battery.
Idles good, runs good at moderate revs, breaks up at high revs 'under load' (which probably translates to when you've got it floored).
Have you ever set the float levels in the carbs to spec? (Hint: let Tony do it for you.)