Yeah I wish I had been around internet when you first posted this. You're likely fine. I've never replaced that seal, and I think there was talk about it on Marlins site because so many people, when doing the clutch or tranny, figure "I'll replace the rear main while I'm in there", not knowing it's better to leave well enough alone, when it comes to that particular seal. And just because you can replace a clutch doesn't mean you're any good at properly seating seals.

But I don't know what the sucess rate it...

I will tell you...doing a tranny/clutch job once and hoping to never do one again is an unrealistic idea. I thought the same thing. I had my Marlin L52HD in and out 3 times.

You may have better luck, but...if you keep the truck long enough, you'll pull the tranny again. Think of everything you do as a learning experience so you can do it faster and with less stress next time. Cause with old trucks like ours, there will probably be a next time, even though you think when you rebuild and replace something "well, that won't need to be done for another 30 years!".

It's rarely worked out that way for me...rebuilt parts rarely seem to last like a new factory part, and once you start taking your truck apart, it's never quite the same again. It's now much more your truck, but once you have things apart, they somehow have a way of needing to come apart more often...