There is actually no difference in the Coronet and Belvedere series B bodies for the period 66-67. They share identical floor pans, unibodies, roof lines, A, B, and C pillars, wheelbase, etc. Same torsion bar length, track, steering, engine location, etc.

The engine choice did switch from the old poly 318 in 66 to the new LA318 in 67. That is normally why they show a change between the two model years.

The engine choices in the B-body for 1966 included the slant six at 225 cubic inch, 318 poly, 273 LA V-8, 361 BB, 383 BB, and 426 Hemi.

For 1967 the LA 318 was added and the poly was dropped, the 361 BB was dropped and the 440 Magnum was added to the GTX, RT, and 660 production B body Chargers.

Small block headers fit the LA small blocks and BB headers fit the Big blocks in the 66 or 67 cars. Hope this helps.