here you go. the definitive test if you're told "i'm pretty sure..."
the top one is for 75' & 76' oem style heat exchangers. 26 3/8" to the inside edge of the muffler bolt holes. i think the one pictured is bent, but the difference in appearance is obvious.
the middle one is the correct one for the early style 2.0 heat exchangers. 19 1/4" to the inside edge of the muffler bolt holes. this style of h.e.'s are flat where the h.e. meets the muffler.
the bottom one is for early style 1.7/1.8 heat exchangers. 21 3/16" to the inside of the muffler bolt holes. this style of h.e.'s have a kick-up at the end where the h.e. meets the muffler.
always be absolutely sure of the type of heat exchangers you have when purchasing mufflers. hope this helps.
k