If you're just swapping stock 351W heads for stock 289 heads, then you're probably going to go backwards. Sure, the 351W heads have slightly larger valves and intake ports, but the compression ratio is going to take a SERIOUS dive. (60.4cc vs. 54.5cc combutsion chamber). Gaining 6cc on a 289 would drop a 9.5 engine to 8.8None of the regular factory Ford heads flow that well... Some are slightly better than others, but all are pretty crappy in stock form. I wouldn't bother unless the switch was being made to an aftermarket and/or ported head.
If you do decide to go through with the switch, you'll have to keep an eye on the steam holes. Steam holes are the small 1/8" or so holes in the deck. They are directly over the bore towards the lifter valley on a 289, but not on a 351W, or later 302's. These match up with the oblong passage on the 289 head, but will not match up to the 351.
You'll need to drill a hole in the deck of the 351 head to match up to the steam passage in the 289 block. 289 head gaskets will be fine to use after this modification.
My advice, save the time and money for a set of aftermarket heads like ported World Jr's with good hardware or stock AFR 165's. The AFR heads have a elongated steam hole to work with any of the production blocks.
My $0.02... been there, been doing that for 18 years now. 
Good Luck!
------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/motorcitymustang/cmml/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
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posted 05-17-2001 12:30 PM
We were never really happy with that 289, that's why we built the 351W with Edelbrock's performer rpm heads.