I run a MSD Blaster 2 coil on my HEI conversion.
I have a M-body, and that what was recommended on Dippy.org

No issues whatsoever with it and it was $50-something dollars 3 years ago when I did the conversion.
Probably can pick up one at a swap meet cheaper if you scrounge around enough, or ebay, they are fairly common items.

Good luck with it!
CC.