IANAL, but the publication of the work is when the copyright timer starts under the '76 act IIRC. However, and this is a big however, because the (re)publisher is not the artist and the pre '76 act did not have automatic default copyright; there is a VERY good argument that it's either public domain or falls under Arkansas local copyright law (which I don't know anything about). I'd say the heirs have a tenuous federal claim at best (again, not a lawyer). But they may have better luck in state court due to common law rulings. Either way it's likely to turn into an expensive hot mess.