11-12 mpg is not too bad for a 350/350 combo. LMC truck should be able to fix you up with most things
My home town favorite is
http://classicchevroletparts.com/ . Between these 2 , you should have the bases covered. If you are looking to replace large body panels like the fenders, etc. --- it might pay off to ask a few body shops where they get their stuff locally, it could save a bunch on shipping for big items
I have very recently starting paying a bit of attention to my '75 Cheyenne as well, so i am a bit cranked up looking through online web sites and things as well --- have fun!!