A '55 291 would be a DeSoto hemi.
They are definitely worth getting if the price is right, I think typically cores go between $500-$1k depending on condition and extras (like a 4 barrel manifold). I picked up the couple that I have for around $500 on average.
The DeSoto's don't have quite as much aftermarket as the Chrysler hemi's (331/354/392) but market is definitely coming around. The nice thing about the Desoto's is that they are all "short bell" blocks. That means that they will exchange transmissions with all of the other hemi's and poly's out there (except the problematic long bell early 331's). It also means all of the transmission adaptors for a 392 hemi will fit the DeSoto, so transmissions are easy.
The DeSoto's also have standard engine mounts on the side, unlike some of the front mount Dodge's. And they're a little smaller dimensionally than the Chrysler's, making them easier to fit into hot rod projects.
