look into what earlyrolla said but i personally would reqire the tacho and just cut the old wiring/plug used if you cant get it working right.
but i do this on the basis that i am going to get the whole car rewired properly when completely finished so not only is it right but also looks nice and neat.
also running off the coil may short the tacho cause it is supplying over 20k volts, thats why it goes on the dizzy as the condensor prevents shorting. but thats with points im no real expert with electronic dizzy.
if i saw it i would figure it out but off the top of my head not definate.