i heard of people doing this is it possible without any mods? isit jsut a pop off the old ones and put the new ones on kind of deal? what kind of power increse would be expected
It's sort of a plug and play thing. The 351 heads have 1/2 inch bolt holes, the 289 has 7/16. So you would have to get stepped bolts (arp has some) or the bolt washer kit that Edelbrock uses with its heads. There is some difference in the water passages. 351 has an L shaped the 289 is more of a triangle (I think). Also if the 289 heads are the early castings then they might be smaller chambers than the 351 (normally they are 58 - 61 cc). Power increase if chamber size is the same I'm guessing 15-20 HP.
Do a search and I'm sure you will find all the info you need on this.
The 351 heads are do-able, you just have to use a certain head gasket and open a couple of passages in that gasket to accomodate the differences in the deck face ports.
Keep in mind that if you are starting with heads that needs work, ie; valve job, hardened seats, new guides, screw in studs, guideplates, etc, and you can't do the work yourself, you can almost spend as much on your factory casting as if you just bought new aluminum heads.
To adapt the head bolts to the larger holes, it's better to just use the shoulder washers that you use with aluminum heads. I used them on a set of 351W E7's on a 5.0.