Can I put a quicksilver 4000 shifter and cables from a 2007 bayliner with a three litre engine into a 1982 260 v8 alpha1gen1 mercruiser leg

Probably needs the cable adjusted. Put the lower unit on and adjust the cable per the instructions for your lower unit.

Yes replace the manifolds and make sure that they are empty of water during the winter

do the plugs spark ? distributer cap electrodes clean inside and rotor tip clean ? compression 100psi or more ? fuel pump working ?

there are 2 cables ! one from the remote controls and one from the engine to the gearbox drive , engine to drive unit has a number on the cable outer but are a standard length as its a fixed distance ! the other cable also has numbers on them as this one depends how long it is to the remotes and routing !

CHECK FUEL PUMP OUTPUT
THEN HEAD FOR THE IGNITION SIDE OF THE EQUATION

contact a marine shop handling mercruiser engines and ask them for the recommended oil for the engine

Some where is an easy pop off plastic cove on the big end of the shift handle. When it is popped and you can see an allen head in the middle of the handle, you can remove the handle and let hang by the tilt wires. under the handle will be 3 or more long bolts that hold the shift control to the console. After you remove them the console will go inside the panel taking the handle and wires with it.

NOW comes the tricky part- the key switch has a power wire going to the control/ Try not to break it. Sometimes you need to loosen the key switch from the dash and allow the wires to follow the control out the back of the panel. Now you can see the cables and remove the faulty one.

we had replaced the engine in our 87 4.3 star craft and when we put the omc cobra stern drive back in we now dont have foward gear, we have nuetral and reverse. could it be the linkage or could it be the shifter cable or any other idea's that could be the problem.

You could be right. Check your idle rpm to see if the motor DIESELS after shutdown. See if linkage and carb setting can be reduced for RPM. Last, check your timing. If it has springs in the distributor you should not see rust powder on pivot points. Binding would keep engine overadvanced at shutdown. Then basic timing could be off too.