I just started my 77 4x4 400 after complete overhaul and both head gaskets leak coolant (LF near intake manifold RR) there is also water in the oil.
THe heads were surfaced and block deck seems flat. I did install a used Performer 400 intake. Fel-Pro gaskets were used.

Does any one have any tricks to make these seal or insight as to why they may be leaking?

I plan on replacing them and coating gaskets with copper spray-a gasket.

Any advise would be appreciated.

 

you should ALWAYS replace used head gaskets. they crush down to a given flatness and if it is a closer clearance in a given spot and then when you put it back on that given spot is a looser clearance, it is going to leak. that's how they work.
did you check flatness with a straight edge and feeler gauge?

 

Head gaskets were replaced with new and straight edge was used on block deck, could not get a .0015 feeler gauge through anywhere.

 

oh... when you said felpro gaskets were used... i thought you meant you installed used felpro gaskets.. DOH! my apollogies.
i don't see why unless you didnt torque down the bolts.

 

Most head gaskets require retorque after the breakin run. Fel Pro "Perma Torque" are an exception but they are not the regular gaskets. I would retorque before doing anything else.

I've seen this exact problem before.