I believe the rochester is spreadbore (small primarys and large secondries) the edelbrock manifolds I see are typically square bore (same size primarys and secondarys) there is a spreadbor to square bore adaptor. It works either way, but I dont think they are optimal. I believe you may gain a little torque as it acts as a spacer but they can leak causing bad idle and i dont think they flow all that great. I could be wrong. I have seen an array of new and used spread bore (rochester) manifolds on ebay relitivly cheap, even the one that fits your mercruiser (assuming it is a mercruiser) http://www.ebay.com/itm/MerCruiser-..._Accessories_Gear&hash=item2ebd02191f&vxp=mtr
You need to add some information.
What carb do you have, Rochester makes 2bbl and the 4bbl Quadra jet.
Do you boat in salt or fresh water.
What year is your engine? Angle and number of intake mounting bolts vary.