To clarify all the above, 64-67 Chevelle 2-door post seats are the same frames, same shape, and same dimensions as a 64-67 Chevelle 2-door hardtop seats. The front bench is split and the backs fold up on all 2-doors (so people can get in the back seat). 4-door front benches have a fixed one-pc back rest.
300's (and maybe also 300-deluxes) have less padding under the seat covers than Malibus and SS's, but the frames are the same.
As to the seat covers with the correct material and pattern for a 300 or 300-deluxe, SMS in Oregon does have the material. And I was told that they will make the covers too (if you are willing to wait awhile and pay a healthy amount). Otherwise, if you have the original covers that a local shop can use as a pattern, then just buy the material from SMS and have them stitched up locally.
Good luck!