Slant 6 swap 170 to 225

Hey guys, I imagine that this has been asked before, but I thought I would give it a try.
The 170 in my '65 Valiant has crapped out (cylinder head) and I have a 225 in a trash '74 Duster. The 170 is mated to a 3 on the tree while the 225 has an automatic. I'm just wanting to swap the engines, not the trannies. What if any problems will I run into?
Thanks!