M= My
S= Spark
D= Disappeared
No offense if you're having good luck (hope you continue to) but the acromyn is accurate.
I know guys that run MSD but they have issues. Others I'm sure have good luck.
CAT calls them "Majorly Sh___y Device".
I ran a D-S II system on the Ranchero back when. Used an OEM Ford E-Core ccoil (laminated ignition coil from a late model). Then I made a conversion wiring harness that allowed me to plug in an MSD 6A in place of the D-S II module. At the time I thought that there was a mild improvement, mostly in cold starts. About a year later I was having some issues that I couldn't quite track down, so I put the D-S II module back on the car. Didn't fix those issues (forget what they ended up being), but I also honestly couldn't tell the difference in how the engine ran. This was a mild 302. The 6A has been on a shelf ever since.
I have run an ignition system using an inductive pick-up coil like a D-S II distributor has, triggering a GM HEI module. Both a 4 pin and an 8 pin GM module. They absolutely, positively MUST have the thermal grease applied correctly. With the exception of some Arctic Silver thermal grease, nothing else will work. Ask me how I know this.
I would say that a good HEI module is a touch better than a good D-S II module, but I wouldn't make the switch without a really good reason.
Since it appears that the OP already has a D-S II system factory wired in his EB I'd use that as-is. With the exception of changing over to an E-Core coil. I think those coils are considerably better than what these trucks were born with. Someone above cautioned about the right type of distributor drive gear for the type of cam (OE roller, aftermarket roller, or flat tappet). There is no forgiveness in this, you must have it correct or the results will not be good.
One of the generic re-curve kits from Summit is probably all that is needed unless you want to send it out to a specialist. IF/When you open up the dist. and remove the pick-up plate take notice of which slot the mechanical limiter is in. There should be a number stamped in that plat next tot he notch. That number is the amount of
cam degrees mechanical advance is allowed by that notch. The opposite notch will have a different number.
Jacobs, there's a name that I haven't heard in years. The guy was a con-artist and not much more. Some of his product was good, but that wasn't intentional. As I recall Chris Jacobs went to jail because of some of his misdeeds and that is why Jacobs no longer exsits.