67-69 Dart swap questions

Must have v8 k frame, good on the 4 speed, 8.25 will last...but an 8.75 will hold up when the boost comes down the road, go with a forged set up and keep the compression no higher than 9.5 to 1 and enjoy.

So is it possible that the car could have a slant 6 k frame in it with conversion mounts for the 318 or can the 318 only be in there with the right k frame? I got VIN earlier and checked it and the car was originally a slant 6, so something has been done to it, just not sure what all has been changed.