i would think 1-3/4 to 1-7/8'' for backpressure.
your muffler shop guys do this ALL the time. They should know if you go to an older shop w/ experienced techs. i have a mildly hot 292ci and run 2' twin split 3+3 to a single quiet #50 walker to a 2-1/2'' out tail pipe. muff well under the bed.
http://www.walkerexhaust.com/support/exhaust101/inspections.asp
http://www.walkerexhaust.com/support/exhaust101/systemBasics.asp
http://www.walkerexhaust.com/support/exhaust101/productsOfCombustion.asp
it's a hard answer as many will say what's best and you may want a loud (rap~rap) or quiet muffler. if the actual muff is placed directly under the cab you're in or lots o noise as well as down the road exh. issues in the cab, so placement is a thought.
That's ALL up to what you want.

Last edited by carolines truck; 09/25/2008 1:14 AM.
Jim & Caroline
The highway is for gamblers, better use your good sense."
Gooday-that's my 1¢ answer due to the lousy economy ~ cause I ain't got - no . mo . doe
Every Shaver | Now Can Snore | Six More Minutes | Than Before ... | Half A Pound for Half a Dollar | Spread On Thin | Above the Collar || BURMA-SHAVE