Being in the industry, I do rember that OMC used center rise manifolds on the 460 King Cobra, I know they were cast iron and heavy, but they were inexpensive. after blowing my first 429, I came across a wrecked (1987 Bayliner) that the boat yard I worked at was cutting up! the motor had 78hrs on it! I stuck the 429 heads on the block and WOW!  after 2 years with TRW 2404 pistons, a stock short block woke up my 79, 19' phantom!  This was the first time I found a true flattop piston (flat with 1 valve relief) in a stock motor! OH by the way those pistons were stamped ford, and carried an industrial part #, and were $68 each my cost. but back to the manifolds, wouldn't these work good w/stainless risers? I/O's use merc manifolds w/after market risers.. I mean they must flow better than logs! I think barr osco still has some... I have a 21 mini day so I'm not worried bout the weight!