I'm a member of the www.67-72chevytrucks.com forum and IIRC, a guy did this with a diesel engine and there was a post on it about 2 years ago. You could ask the same question over there -folks are very knowledgeable and friendly and I'm certain you could find an answer to your question!

1968 Camaro - 250 (No, I'm not gonna drop a 350 in it!....Jeez!)
1968 C-10 with 2 extra cylinders