I am in the process (and what a process) of putting a 454 into my 71 K5.  We have had to do some fabrication to the mounting cross member, etc.  We ran into issues with the clutch linkage not clearing the headers.  We were told we could replace the linkage with a newer type but it would require a replacement of the bell housing as well since the newer versions mount on the bell housing.  Anyway, we went to the junk yard and picked one up but the bolt patterns did not match.  Is this going to be a transmission specific issue?  Has anyone out there done this kind of conversion?  

If you got a bellhousing for a NV4500 the bolt pattern for tranny is different than a NP465 plus there was two different bolt patterns used by N4500 too. Approx 92 to 97 was one pattern and 97 to present another.