exhausting & its a hot topic
Boy that was a bad pun... Anyway, back to the advice. Silverblue offers good advice I reckon, so I wont repeat that. And as he mentioned, there is way too much BS information around regarding exhaust systems.
For a mild 289 such as yours, a street driven car, you need all the torque the engine can make. Torque makes the car nice to drive at lower RPMs where you will be driving on the street (mostly hehehe)
Do remember to use a crossover pipe after your headers, now sometimes called an X pipe too (due to the shape) That will make the "note" sound better and increase torque. Use quality mufflers, do not use glass packed straight through "hot dog" mufflers. If you have time and the money, use header wrap or ceramic coating to keep the heat in the exhaust system, it makes the headers more efficient and reduces under hood temperature (lower temps helps increase power too) If you do the header wrap, paint it with the heat proof paint to make it last longer.
I agree that Tri Ys should produce more torque, it really dpeands on how good both the headers are, and longer, narrower header tubes will make torque at lower RPMs.
The exhaust will make a huge difference to your ride, putting a big smile on your dial.
Oh, just in case you are wondering, my exhaust is twin 2 1/2" and I am still hunting a good set of headers. (Hooker I think)
cheers n beers, Mick.