pedal assemblies for 78 chevy 4x4

I am still working on my 78shortbox 4x4 and decided to convert to a manual, bought a sm465 and 205 my question is about the pedal assemblies, Having converted a 65 gto from auto to manual all I did was get pedals out of a chevelle and they went right in, I was told for my truck that I must change the the whole assembly instead of just the pedals and what I mean by that is the channel up under the dash that the pedals hang from is this correct? Does any one offer pedal assemblies and linkage through  aftermarket?