Floor pan

In 1966, the US model Valiant body-style (top pic, short wheelbase) was not available in Canada...just the US body-style Dart, rebadged as a Valiant (2nd pic,long wheelbase) for Canada. i believe the '66's are the same as the '65's, the longer wheelbase model (CDN Val/US Dart) has a filler panel that goes between the center/front floor pan and the rear under-backseat/kickup area pan to make the longer wheelbase. By omitting that filler piece, you can use the longer WB pan in the short wheelbase car.

"Custom 200" means its a Canadian car (also 2nd pic)...aka: rebadged US dart.

3rd pic shows filler piece used in long WB cars.....

Here's a great page that may explain the differences better than I...

http://www.valiant.org/canada.html