I did the HEI conversion on my '67 Suburban with a 292. A little research and background helped - search this site under the electrical forum - might also be under the FAQs. Also, take a look at
www.madelectrical.com - lots of good background info on engine electrical, including updating the alternator/regulator system if you're so inclined. I searched the local wrecking yard for a '75 or later (I don't know to what year) Chevy six (250 is ok). The earlier HEIs had separate ignition coils, the later ones have the coil on the cap - a little cleaner look in my opinion. I took the plug wires too for an easier swap, new ones probably wouldn't be a bad idea. You'll need an upgraded, higher-amp, 12V supply to the coil. Again, search the electrical forum for ideas. I used a relay system which I'm told was over-kill but so far seems to do the job. Good luck!