340/360 Heads on a 318

Might have just found a great deal on some 340/360 heads from a friend of a friend.
Completely rebuilt and hes asking very little for them.

He told my friend to tell me that it would be a totally worthwhile swap for my 318, and I've heard of this before, but I'm simply curious:

What advantage is there to adding 340 heads to a 318?
My 318 runs great now, completely stock except for an Edelbrock Streetmaster 318 intake and Edelbrock 650 Carb, and I'm just curious what the difference will be if I pony up the cash for these heads.

Thanks FABO! :read2: