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Somewhere, waaay back in this thread, we had an earlier discussion about mixing lines.
There are good points about keeping lines separate, and good points about mixing them. So, it kind of depends on what you want to do.
Mixing lines will increase genetic diversity. This can be a good thing, BUT it means that your offspring will have more variability. You may get some very good birds from the cross, but you may also get some very bad birds from it. You will have to do a lot of careful selection to create a new stable line of your own.
Keeping the lines separate, OTOH, may give you more uniform offspring. BUT, if your individual lines have faults, you may be stuck with those faults. There may not be enough genetic diversity left to let you select away from them.
So, there is no single best way to go. You have to decide for yourself -- do you want something more predictable, or do you want a challenge that might lead to something really interesting in the future?
It is my personal opinion that most of the "lines" of Marans around aren't really isolated enough to make all that much of a difference between keeping them isolated or crossing them, in any case. But those are the general considerations when you're trying to decide what to do!