I was given a pair of monster flowmasters... free... put them behind my 305 in my 78 firebird. they are 3" inlet / outlet muffs, and are welded to the collector, bolted to the hedder. Used some scrap ( AKA GARBAGE ) 2" pipe to route it out the sides in front of the rear wheels. The short length of 2" seems to give me all the back pressure I need, and it sounds / performs great.

Oh, did have to weld some scuff plates to the bottom of the muffs though.