I remember asking the same thing awhile back, and wound up going with a Duraspark distributor.
I'm going to run this set-up:
Duraspark "Blue Grommet" ignition module (1981 F-15 w/351M) - O'Reilly P/N: CBE7P $35.99
NPD Duraspark conversion harness - NPD P/N: 14289-40A $113.95
Cardone distributor (thru Summit) - 100-302893 $63.95
Duraspark II cap & rotor (small cap to fit under Ram Air) Summit P/N: SUM-G5208 $14.95
Accel Super Coil - Summit P/N ACC-8140HV $53.95
The only "expensive" part was the National Parts Depot conversion harness, but it has the resistor wire and is a complete bolt-in/plug-in unit with the right plugs, sockets, etc. I also like the idea that you can pick-up pretty much everything from a local parts house because it's essentially the same stuff as you'll find on just about every Ford truck and 'big car' from the '70s through the early '90s (rather than wait for a mail-order MSD ignition box or distributor stuff).
This is pretty much the same set-up I have on the Jeep, without the harness (Jeep already had it), module (Jeep already had one) and distributor (Jeep already had that, too). Really woke up the Jeep's anemic 4.2L (factory 125hp) and I'm running Champions with the spark plug gaps at .055" - with no more 'rich' condition.
Here's the link I found talking about it from VMF:
http://www.mustangsteve.com/conversion.html