Since you already have the wiring setup for the 6A, how about sending the existing unit to these guys for a rebuild?
This way you can keep your wiring and save some money on buying a new unit. Techwest will rebuild your existing MSD unit.

http://techwestracing.com

""If you have a failed MSD ®6a,MSD ®6al,MSD ®6t,MSD ®6aln, you have only to pack up the failed MSD ® ignition box and ship it to us. We do ask that you drop us a quick note to give us a heads up so we can be looking for your MSD ® ignition box. Once your MSD ® ignition box arrives here at TechWest Racing Services we will troubleshoot your MSD ® ignition so we can provide you with options to repair your failed MSD ® ignition. The average repair charge 90% of the time for the above failure runs $90.00 to $125.00 plus shipping charges. Anything above that will be charged at the normal shop rate of $55.00 an hour plus parts."