Does anybody know how to replace down-tube shift levers with those that attach to the end of aerobars? I have 8 speed 600 (indexed). what would the new levers cost, or would I even have to buy any? Can anyone recommend a good tri setup that I could attach to my roadie?
I've run a setup like that before, with the shifters at the ends of the aero bar. The style of shifter you're referring to is often called a "bar-con" shifter, or a bar-end shifter, and Shimano makes a complete set which include cable-housing stops that go where your downtube shifters are presently mounted.
You can also attach a standard Shimano indexed downtube-style shifter to these bar-end mounts, in place of the short shift levers that come with them. For this reason, if you wanted, you could pick up a 9sp set, then put your 8sp lever on and keep the 9sp lever for the future. Here's a link to the 9sp version, cables and stops not shown: click me
Having built my share of Quintana Roos up 'til about 1999, the typical shifting setup then was indeed the bar-end shifters at the ends of the aero bar, or else a Syntace shifter mount on a Syntace aero bar, with standard downtube levers on the Syntace mount.
This shifting setup is not ideal for everyday riding in a stop-&-go situation. Reaching for the end of an aero bar to downshift, while braking for a stop light that just changed to red, is Not Fun, and I would know If you want a practical compromise, consider mounting these shifters where they are meant to go (in the ends of drop handlebars) or consider some integrated brake/shift levers if you run a bullhorn bar.