502,871 blower.What type of ignition system should i be looking for?Something with a boost timing retard function.Currently have standard merc thunderbolt IV.Can i use my existing distributor?Any help greatly appreciated.

Do a search on Crane HI6M. I think it will suit your needs. There have been many posts regarding the Crane system.
http://www.cranecams.com/pdf/90006462b.pdf
I have been using one for 6 years with no problems.

Looks good to me.Thanks For that BadDog.Where do i get one?Summit Racing?

Anyone interested in this,i bought a Crane ignition and its fantastic.Multiple timing curves,rev limiter and boost retard.Great.

I have used a few too and they are fantastic. A lot of features for the money. Oh, and they seem to be pretty durable - but won't know on that until one actually breaks. LOL.

I would run MSD. You can couple your MCM dist with the MSD box with boost retard. Or run MSD distrubutor

The Crane HI6M is wired for the Merc distributor and harness.It has adjustable boost control built in.No need for additional hardware.,rev limiter and 8 different timing curve choices all based on the Merc racing engines.It is the best.

Simple is good.
Get a V6 Tbolt module. Set your total timing. You get to use your current distributor.
If you have fowling problems at idle, add an MSD box.

Crane system hands down. They did their homework on the marine stuff.

Thats what I would do, except I would opt for a crane ox.