Switching out a good slant for a stock 318 really ain't worth it. Less mpg and not much more power, plus it is a bigger pain in the *** to work on.
Cut the head on the 6 about .100 and put at least a 2bbl on it, you will be quite close to a 318, put a cam in it and you can easily be more powerful then a stock 318.
Now swapping the 6 for a 360 could be worth it. Honestly if I was going through all the trouble to swap a V8 it would probably be a big block, or a 340/360 as an absolute minimum.
Just my thoughts.
TopHat