Van exhaust?

There is no special 2-1/4" exhaust manifold from a Dart Lite. The Dart Lite/Feather Duster got the same exhaust manifold as every other '73-up Slant-6 engine, and its outlet size is 2", though it can be honed out a little if you want to.

Those "turbo" mufflers like to set up a drone that will cut right through you with an inline-6 engine, and Murphy's Law dictates it'll be at its worst at exactly your most common cruising speed. Get a real muffler instead: a Walker № 50051 (Hemi Ram truck muffler). Huge capacity for excellent flow, made of stainless steel for nice long life, excellent sound control at all engine RPMs. Use a 2-1/4" headpipe off the manifold and into the muffler. Use a 2" tailpipe out of the muffler, and if you want it extra super quiet, put a resonator in the tailpipe. If I'm rich, I like a Flowmaster HP2 stainless unit with 2" in and out. If I'm broke, I like a Walker № 17198 4" round can-type unit, 2" in and 2" out, with offset outlet next to a proper hanger bracket.

Tailspout outlet between 45° and 90° outward, and 45° or more downward.

With or without a resonator, this system will have a whole lot less restriction/backpressure than the original-equipment system, but won't drive you bonkers with noise (loud exhausts are fun for maybe a week, then they get old).