Will it fit?

Actually, I wasn't sure, hence my question mark. Even with the frame rails there, it seems like it could go in at an angle and sort up up and wedge it in there, but maybe not. Seems like it would be a bigger hole than the trunk opening anyway.

But, this article that may be helpful implies different. It's a '67 Valiant, so should be about the same. If it hadn't had the large trunk opening, it says they may have opted to install a 2 piece pan instead of 1 piece:

http://www.moparmusclemagazine.com/projectbuild/mopp_0311_1967_plymouth_valiant_trunk_pan/

And, different car, but here's one where they install a Superbird trunk in two pieces. So, it looks like your answer could be no:

http://www.moparmusclemagazine.com/howto/5147_trunk_floor_install/

Hope those articles help at least, if no one replies who has actually done this on a Dart.