Options for Replacing Slant 6 in 65 Dart

A 273 V-8 was available from the factory in 65, so that would be a logical first step. 273's and 318's are outwardly identical, so a 318 will bolt in using factory parts. 340's and 360's are substantially similar, although the mounting points on the block are slightly different. A 440 or any other big block was never offered in a pre-67 A-body, so there are no factory parts to install one. It can be done, but requires significant fabrication (custom headers).

In any case you'll need the corresponding V-8 transmission, a more robust rear axle, better brakes and so on. Plenty of us are replacing slant sixes with V-8's on this site (I'm swapping a 318 into my 65 GT) so there is plenty of info out there. Search your way through old posts and forums and see what others have done.