how tough to put 318 in '64 Dart GT with Slant 6?

The 318 will fit the engine bay just fine; the K-frame is the same for 6- and 8-cylinder engines in the '64-'66 A-bodies; you just need the V8 engine mounts. You cannot bolt the V8 to a Slant-6 transmission; you'll need a V8 transmission. You'll also need beefier radiator, brakes, and suspension. No difference in steering column.