Hi, I see you haven't got any reply yet so I wandered if you rechecked your cam timing yet? I worked on quite few Fords and didn't have problem with setups for timing.It sounds like as you suggested the cam is out.I wouldn't yank the engine out yet before recheck TDC both intake and exhaust valves closed position.Just removing the valve cover on the right side at number one cylinder looking toward the front of the boat you sould be able to tell.If your distributor is out 180 degrees that's were all the backfire and spitting coming from.Another thing despite the fact lot of people put timing TDC I tried all variables on timing -4deg 0TDC 4ATC 6ATC {I did this same day ,took awhile doing timing cover removal each time}top speed was achieved at 6degrees ATC.Couldn't tell much difference in loss off hole shot speed since all jets reach full rpm almost instant.I guess if you into racing it's a different matter.My boat is a 20 spectra with a JG bowl AA impeller 429 cobrajet with the cast iron SCJ intake KB pistons fordmotorsport marine cam 493/502 lift 780 holley 72 primeries 85 secondary jets vacuum secondary.The boat is approx.3000 pounds,engine max rpm 4850 one person, tank one third full, 60mph on GPS at sealevel.I have tried Offy dual plane manifolds but the SCJ cast iron runs best despise the fact the intake opening is much larger on the cylinder head side.I think Edelbrock or weiand single plane is best suited for the big block fords.