Son,
SAAMI standard for the .45-70 L.O.A. is 2.550" All bullet moulds I have, which is several, ranging in wt. from 175 gr. to 550 gr. when seated to crimp over the top band come real close to this figure. Weight in itself has nothing to do with LOA. Even the Sierra, Hornady, Remington and Speer bullets designed for .45-70 have the crim cannellure in the proper position to give this LOA. Heavy jacketed bullets designed for .458 Win. or any of the magnums may be too long, but they are too tough to give proper performance on game at .45-70 velocities.
To make a slight correction, the light bullet comes up much shorter than 2.550. All others are or can be seated to this length, some in a crimp groove and some over the top band. Older moulds designed when black powder was still commonly used generally will not have a crimp groove as ths keeps the bullet from being pushed in and the full case loads which must be used with black powder fulfills the same function.
To make a long answer short, heavy bullets are not a problem in the Marlins, regardless of weight, unless the nose to crimp length is significantly longer than .450"