I don't use the 'Off Road' or 'Marine' versions of the MSD 6 module, they are 'Potted' with an epoxy.
That epoxy is there to keep moisture out, and keep the little electrical bits on the circuit board when you pound on it.
The epoxy also makes the unit NON-SERVICEABLE if you have problems with them.
MSD had, and as far as I know, still has a very good warranty program if something goes wrong.
I've had MSD modules on my vehicles for years without any issues, just be careful about your mounting point.
Don't get them too close to engine heat/exhaust,
Don't get them someplace they are the FIRST thing to go underwater.
Use some rubber shock mount feet under them to help absorb some vibration, and keep the module up so air flow can get under it to help cool it.
I mount mine in the cockpit to keep the crud out of and from under the module, and that works pretty good.
Behind the dash panel is a good place, if you have room...
I ALWAYS get faster starts, more MPG's (with a little tuning) and I don't have to babysit the engine when I first start it with an MSD like I do with the factory ignitions.
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Since I've been in water up over my hood (sitting on the back of the low back seat, driving with my feet!),
I mounted my module on the side, inside of an old ammo can, on rubber feet.
I mounted the can UP SIDE DOWN behind the dash. (No lid on the can)
The can works to make an air bubble inside, so my module doesn't get wet.
I took pains to make sure the can was air tight even with the screw holes in the sides (sealer around screws) and it works fine for what I'm doing...
I expect since the module is mounted edge-ways, I will eventually beat the small components off the circuit board, but it's got more than 10 years on it with no problems,
So to replace every 10 years or more, and you can find them used CHEAP,
On an 'Extreme' use vehicle, I don't think that's unreasonable.