Engine swap

I am new to this site- I look forward it!
I have had Mopars for the last 40 years, all small & big block cars. I have been asked to swap a small block for the stock slant 6 in a 75 Dart for a friend.
I know I will need to get a different auto trans because of the bellhousing mounting pattern.
My question is this: Will I need to replace the "K-member" or just the motor mounts?
Any other needed info on the subject would be great.
This looks like a great forum!!