What Casper said... Unless you go w/ a "cat-back" exhaust system, you're going to just have to find a muffler w/ the correct ID/OD (inside/outside) diameter pipe & have it welded up. Any competent muffler shop can do this w/ no problem.
FWIW, I just had a Dynomax "SuperTurbo" muffler installed on our 2000 4R. Supposedly one of the quieter turbo mufflers on the market (very little drone), & claims to outflow the equivalent Flowmaster model. Barely noticeable at idle, but has a nice healthy growl as you move up in the RPM range. Gives the 3.4L much needed tone, IMO.
Re: seat of the pants, hard to tell but it does seem to rev a tad quicker. I forgot to reset the PCM after we had it done, so maybe the computer hasn't yet learned to take advantage of the reduction in backpressure. For <$150 (incl. cost of the muffler), a worthwhile mod IMHO.
Her4Runner