Just get an aftermarket 'HEI' with external coil if your worried about room. It will be as small as any stock distributer.
Or better yet. Just go out and buy a points to HEI conversion kit. These things work great and are much cheaper.
If you read up on all the fuss about aftermarket distributers/coil/modules w/ their high spark and duration(by the way, long spark duration makes no sense to me...I think id prefer a strong blast of spark over a weak drug out spark any day), you will find that they basically do you no good unless you have a hopped up engine that is capable of spinning 7500-8500 rpm.
Ignitions will use the minimal spark and no more that it takes to jump the gap. I realize that I could be exiled by all the people out there that have blown $1000 bucks on a super duper **** ignition that could light your cigerette from 20 feet, but im willing to take that chance.
Thats where these conversions come in nicely. They usually dont require you to run a true 12v to them (they usually only require 9.6v-10.something)...I believe the wire that is feeding your present distributer is a resistor wire, which knocks down the voltage into that range. And lastly, you get to keep the origional look (if u care).
Anyway, thats my 2 cents.