Slant 6 to big block in 68 Cuda

Okay here's what I have. 68 barracuda
Already have converted the front and back to big bolt pattern
Disc breaks
no power steering or breaks
currently has a 318 but was a 225 slant car
8 1/4 rear
-I have a 360, 383, and a 440 all need rebuilt
Also have a 727 trans

Do I need to swap the K frame? Will the trans crossmember work? Do I need a eight and three-quarter rear? Never done this I need any help I can get. This car will be just a driver with power
Thanks