318 differences ??

I don't do too much with small blocks so it might seem obvious to some but.....

I have a running 318/904 out of my 68 coronet ....seems to be the original, everything is late 67 dated which corresponds with the build date of the car.

I am not going to use it so a neighbor guy up the road wants to buy it to use in his 75 duster. The duster has a small block K-frame but didn't have an engine when he got it.

So our question is: how much different is a 68 318 from what a 75 318 would be? would it be better to hunt and find a 75 318 or close for the duster?

would it be a bolt in deal? anything to look out for?