I picked up a g35 6spd coupe cheap with a potentially blown head gasket as the rad was empty and it has thick milky white substance on the oil cap. I've done compression checks and leak down tests and all the cylinders are within the 10% loss.. I'm wondering if it's worth it to tear it down to try to change the head gaskets or put in a used engine with low KM's . I found an engine that has low kms but is from an 03 sedan manual . Are these engines the same? Direct swap? I've searched all over but there seems to be no definitive answer. Any help would be great.

 

If your coupe is an 03-04 auto/manual then I'm quite sure they're the same.

I think it's something like:
03-04 auto/man are the same for G35 coupe, sedan and 350z (DE with variable intake timing)
05-06 g35 coupe, sedan and 350z are the same as above once they are auto.
05-06 manuals are DE rev up with variable timing for both intake and exhaust
07+ have the HR's

Also for reference, my 04 g35 coupe auto has an 04 350z manual engine

 

To clarify the above all is correct but the 07 coupes are non-revup in the automatic and rev-up in the 6MT. The coupe didn't get a new motor until 08. 07 sedans are the HR.

 

Definitely tear the heads off first and see if a gasket is blown. If a head gasket or gaskets are blown and the head isn't warped or anything else visibly damaged, just clean it up and put new head gaskets on and flush the coolant. Head gaskets themselves are cheap enough that it's worth trying the replacement if you are doing the work yourself. It would be so much cheaper than buying a potentially unnecessary whole new motor and swapping that in.

I think a whole new set of all hardware for the head gasket replacement is only like $70 and it includes all gaskets for everything, new bolts and everything else that should be replaced along the way.