I suppose a fresh rebuilt G can put a hefty load (high current draw) on a starter (especially if starter is showing its age or going bad) which may reduce battery voltage down quite a bit causing a low energy spark while cranking, although no way sititng here I can say thats your problem.
Things which can help (if? the above is the problem) may be a heavy duty battery such as say 1000 CCA minumum,,,,,1 or even 0 Gauge Battery and Ground and Starter Cables,,,,,,Try running the ground right to a starter mounting bolt,,,,, Use 12 gauge wire in all the primary ignition circuit (ammeter to switch to coil to distributor).
Are the points and condensor new or in like perfect condition?? as a weak or bad or open condensor causes a weak spark CHECK THAT CONDENSOR,,,,,,,,,,,
The ignition switch may be resisitve TRY TO HOT WIRE THE COIL DIRECT which by passes any bad loose resistive contact switch problems.
Are you at the original POS or Neg ground??? The coils polarity should match the battery. If at Pos ground switch feeds coils - and Pos wires to distributor, reverse that for Neg ground
Since youre running a 12 volt coil theres no ballast resistor hid somewhere thats still in the ignition circuit is there??
Does the distributors advance system work right?? There shouldnt be any advance while cranking then as RPM increases the weights pull against the springs to advance the timing.
TIMING is she firing right near TDC while youre cranking ????? You need to check the timing ESPECIALLY the start/cranking timing
STARTER if the starter is going bad it can draw excess current which drastically drops battery voltage while cranking, sure the starter is okay????
ONE LAST THING its possible to use a 6 volt coil and a ballast resistor with a ballast by system which apples unballasted voltage to the coil while cranking, that really improves start spark energy.
Theres more but I have other things to get at lol