I took my TR3 to a local independant muffler shop with a similar request. He assembled an exhaust system using two "Cherry Bomb" glasspacks that I feel is just about perfect. Definitely audible at low speeds, but not too loud to talk over; and at higher speeds it more or less disappears into the general wind and road noise.
It's been a long time since I had a stock system, but as near as I can recall, the new one is quieter than the reproduction stock system I bought from Moss long ago.
Best of all, the cost with installation, tax & everything was under $150, and he took care of it in about 30 minutes. I didn't even have to change clothes!
If you look close, you can see the rear muffler peeking out:
http://s258.photobucket.com/albums/hh260/TR3driver/TS13571L/?action=view¤t=DSCF0013.jpg