Rule of thumb iv always gone by...Seems to ring true even more for inline motors.
up to 200hp... 2" duals at the most, maybe a 2 1/2 single
200-350hp... 2.5" duals, 3 inch single is usually good
For your setup i would probably do a Y bolting up to your collectors, with a decent sounding muffler that still provides backpreassure. Seems the inline motors like a bit more backpreassure to run the way they like to.
On my 1975 292 (holley 500, offy intake, offy headers, a little internal work) I run 2" duals, but might be able to get away with 2.5. With my 82 k10 with a 292 I run 2" duals as well, but I think the "big" exhaust on an anemic yet torquey stock emissions 250 could be improved by going to a single or smaller duals, as anything beyond 3000rpm suffers.. My v8 trucks definently require a bit more breathing and their running characteritics benefit the bigger pipes better.
