You might want to check out "Electrolytic rust removal." It's cleaner and there's no chance that a forgotten piece of blasting media will dislodge and do damage later. You can get creative by putting the sacrificial wire inside the manifold and supporting it with plastic before submersion. Acid or lye will also clean the inside but you really need to be set up to work safely with these hazardous chemicals.
https://www.google.com/search?q=elec...utf-8&oe=utf-8
http://www.instructables.com/id/Elec...val-aka-Magic/
http://www.stovebolt.com/techtips/ru..._derusting.htm
Hot tank cleaning is likely offered by your friendly, local engine machinist.