I've seen this question before, unfortunately don't remember all of the details. Seems to me that changing the cam was one of the things suggested, besides replacing all the marine gear. The marine engines are designed more for steady engine RPMs. Here is a thread I found on Stovebolt.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=003074#000000 Here is the thread I remember. Did a search of the archives using "marine".
http://www.inliners.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=001661 Larry
68 C20 327 daily driver
63 C20 292 Camper truck
65 C10 ? Custom Cab AC,PB,PS,$flasher
