cat length vs premuffler length?

my '78 needs a new exhaust, but only from the premuffler back. MSA has a 130 dollar kit for the cali cars with the catalitic converters, and it's a direct bolt on for those. I emailed them and they said it might work with some changes, but i was wondering if the length of the cat, vs the length of the premuffler, from flange to flange, is the same, and if the flange itself is the same. if it is, i think i'll buy the cat-back system they have for the 280's