I've got a mid 70s 455 with J heads on it that I'm putting in my 70 Cutlass, while I was going through my stuff that was in the trunk I found a set of C heads,would these be worth rebuilding and putting on this engine?

 

The C heads are better flowing heads. Check if the intakes are 2.07 or 2.00

 

C heads port out well.. and with a good valve job with the right angles.. you can make them swallow a 2.14 intake valve and get them flowing 278@650 with fat low numbers .. flowing more than stock EDE heads.. but you'll invest probably 30 some hours into them..

 

Stock for stock you probably wont see a lot of difference. The C heads will probably run a little better.