i dont know of any stock 289 heads that have 49cc chambers, most have between 53-56cc. measure yours to be sure. smaller chambers are better because you get a better compression ratio.
as for porting, yes you can pick up some power by opening up the ports. you want to match the port opening with the gasket on both the intake and the exhaust. you dont want to try and open the ports up all they way down though as you may run into the water jacket and ruin the heads. but you can open up the ports and blend back and 3/4" or so into the port without issue, and use the dremel to clean up the rest of the port to improve flow. there are articles and videos around to show you how this is done, though you will have to find them.
with a decent job you can pick up 25-30hp with about 8 or so hours of work for basically the cost of the dremel and some dutting heads and some sanding rolls.
a few things here;
1: safety. wear some breathing protection, since you are grinding on cast iron and that will damage your lungs to breath in that dust. surgical masks do a good job.
2: take your time. remember that the port walls get thinner the closer to the valves you get. to be careful. if oyu do break through the port wall, dont panic. often times you can use some high temp epoxy to seal the breach.
3: let the tool do the work, dont force any thing to happen. you will only break tools.
4: do not polish the ports, engines make better power with a slightly rough port wall so the fuel mixes better with the air.