Any Parts crossover between Dart and Polara?

I have a 65 C-body and a 65 Dart. Some interior parts appear to be the same - door handles, headlamp switches and such. Nothing in the suspension, steering or brakes would interchange except the steering gear and p.s. pump. Charging components are the same (alt, Vreg). The bulkhead connector is the same, indeed I put one for a 65 Fury (ebay) in my 64 Valiant to get the better 65 design (but not for your 66). If the same engine, then everything but the motor brackets/mounts interchange. I don't know about the throttle linkage, but "should work". My 65's are both cable throttles. I know a B-body radiator is slightly too tall for an A-body, so probably also true of C-body.

Little in the drive-line or rear would interchange. The C-body 727 tranny is longer than A-body ones. A few C's had 904 tranny's, which I don't know about. Perhaps you could shorten a C-body drive-shaft to work in an A. The tranny output side should be the same (though spline count varied over the years I think) and you can get U-joints for the rear that adapt bearing sizes.