1. 289 heads aren't impressive in flow, but you have them. The only heads that did flow were the 69 351W heads though so choices are'nt abundant.
2. Watch compression. You know the shortblock advertised compression, but do you know the cc's of your worked 289 heads? With 44 year old heads, make sure that due to possible machining that the intake still lines up correctly.
3. It'll bump up the 302 compression and most likely out perform a 289 with the roller cam and look to all but the concerning eye to have a 289 in the bay.
I always lean toward performance and if this is just to get a car up and cruising, then go for it and ignore my concerns....it will "work".
The simple fact that they're ported is going to make a difference. The 66 289 heads according to the factory are 55 CC or about 10 CC less then the 87-93 5.0 heads. Should make for about a full point more. Final number? Crap shoot IMO. The only big problem, make sure you have your tappets in before the heads go on. Once the non roller heads heads are installed on a roller motor, you will not be able to remove or install tappets. You'll need the non roller, cast rocker style pushrods as well besides the correct rockers, rail or non rail rockers. The non rail rocker heads have guided push rod holes.