I am almost certain that you are going to find that your original Marlin bead is going to be both too low, and too big. (I've been thru this lots of times!) If you can get hold of a Brownell's catalogue, you can can them on the phone, tell them the exact receiver sight you have, and they will actually tell you the exact front site you need, including height and bead size.
I have found that the smaller the bead size, the better.
As for a problem with it covering up the target, I have not found this to be a problem on deer out to 150 yards, or small game out to 50 yards. Since you are shooting a big game cartridge, I don't think that you are going to be shooting many rabbits or squirrels anyway.
The very best front sight as to visibility is one of the Tru-Glow, blaze orange sights. Absolutely incredible. They look like they glow! But, they are somewhat delicate, consisting of a plastic rod insert. Happily, they sell extra rods with the sight, and you can carry one in your kit, in case you break the one on your rifle. I have never actually seen this happen, but it is one of those things that hunters tend to be careful about. These now come as standard equipment on many expensive muzzle-loaders.
If you are a deer hunter, you are going to quickly find that the best thing to do for the rear sight is to unscrew the peep aperture, leaving the larger hole, and just look straight through it. This is called a "ghost peep" and is very very fast! Lots of light comes in through the hole, and you can see that front sight really really well. Your eye will naturally line up the bead site into the center of the hole, almost instantly. It also gives you an extra ten minutes or so of shooting time, as the sun goes down.
Best,
Mannyrock