Took a chance a while back on ordering a flat '71 hood for an mpc roadrunner from 'rdsxfn' (or something like that) but wasted my money. Poorly done with a gouge across the cowl grill as well. Wasn't worth the trouble to send back.
Here's a '55 I built at 14 in the 70s from the blue car box art kit. The Buick wheels came from the '56. Had no problems to speak of with it and am amazed at how well that testors rattle can black laid down and remains in good shape. Just picked up a builtup to use for restoration of a few missing/broken pieces.
The flat top sides started with the '77 trucks. Ford used the same style and width flare top beds from 53-72. In 76, after a three year break, they reinstituted the short flareside box, but it was wider (just a shortened eight foot flareside box) to match the new rear axle width brought out in 73. The sides proved too tall with the flare so they flattened them for the 77 and later trucks. You basically have your choice of four boxes: the 1/25 mpc and amt '53s, the 1/25 revell '56 or the 1/24 monogram '55.