Hey fellas, just to chime in and drop my .02, it can be done, I have it. Now, I am by no means an expert or engine builder, I just read a lot and listen to what others have mentioned or done.
I did not build the engine, but I set the parameters I wanted and it was basically built to what Shelby American was doing in the 60's with the 289. When I purchased the car it had a lot of vintage parts that were added by the original owner who I am told worked for Shelby American.
This is what the engine had in/on it already: Comp Cams Magnum flat tappet 294s (248 duration/.560 lift) cam and matching springs, Comp Cams 1.6 roller tipped rockers, windage tray, Comp Cams timing chain, stock heads, aluminum high-rise intake, dual 600 cfm Holleys, dual exhaust with H-pipe chocked out in back by GT trumpets, MSD 6A box w/Pertronix ingnition and coil.
What the shop did: Basically rebuilt the engine. Like mentioned, bored it out to .040, ported and polished the factory heads, Serdi valve job and installed Manley 1.84 intake and 1.60 exhaust valves and port matched heads to exhaust gaskets(Doug's tri-y headers), Manley poly-locks, cleaned up intake ports (combo of tunnel port and open plain intake), shot-peened factory rods and crank, crank was chamfered and polished, Melling hi-volume oil pump, ARP bolts everywhere, Speed Pro forged pistons, Clevite 77 bearings, compression at 10:1 and balanced engine assembly. I am using the stock distributor that was sent out and rebuilt/recurved to the specs of the engine.
Holleys were sent out and rebuilt (don't have specs on those) and they are vacuum secondaries. It looks like the choke plates were milled off in the past. The car is all manual.
I had them dyno the engine and it put out 375 HP @ 5800 rpm and 346 lb. ft. @ 5400. On the chassis dyno it put down 317 HP and 297 ft. lb. all running through a 4 speed toploader, aluminum driveshaft, 8 inch rearend and 91 octane fuel.
I know I could have used a multitude of different combos and perhaps gotten better results, but I am quite content with what that little 289 can do. It idles at around 750 rpm and has no problems taking me to 6K. By all means my car is streetable, it is a weekend car, cruise-in car and soon to be open track car. I probably have around 2-3K miles on it so far and it is absolutely a blast.