I'd go for the optimax if it's not too expensive. Those old 3.0L 225 mercs were fuel thirsty slugs unless you have them worked on. There are shops like simonsmotorsports that can wake them up. Might be worth calling around . . . There was also that company in FL who was working on a bolt on diesel powerhead for 3.0L mercs.
http://www.simonmotorsports.com/page15.html