[84LX89GT]

From what i've read i think they flow a little bit better than the E7TE heads especially on the exhaust side (but only if the 289 heads don't have bumps in the exhaust ports for air injection). I read an article about a low buck 400 horsepower naturally aspirated 302 that used 289 heads with 1.8?" intake and 1.54 exhaust valves, but the heads were extensively hand ported and polished on the exhaust side. So basically they're pretty close flow wise to the E7TE's and the combustion chamber shape is about the same as well (fairly shrouded valves) which is another area that could be worked for more power.

If you're going to have a carbureted engine in your car, the 289 heads would be better than most garbage 70's and early to mid 80's heads as well. Another option that's fairly cheap as well is pre-'74 (i believe) 351W heads because they already have the larger valves and larger ports and if you can find some without the thermactor bump in the exhaust they flow pretty well for a stock head.

So TO ANSWER YOUR QUESTION.....i don't have any flow data or anything, but i can find some, i believe that the heads you mentioned MAY flow better. If they're already set up with better springs/performance valve job, etc. then they're probably worth the trouble of buying.
....i'll try to find some flow data for you