Check the head studs at the case see if they have pulled, you should be able to see the gas discharge from the cylinder on the inboard surface of the head this will tell you which cylinder is leaking.
If the head has warped the only real way to check is to take it to a machinist to be measured.
If the head is not seating on the barrels evenly the only way to tell is to take the head and barrels off. measure the barrel length from the seating surface on the head (inside the barrel).
If the barrels turn out to be at different lengths from the contact face on the head it is possible to buy shims that go at the base of the barrel to correct the discrepancy.
The only time I think it would be the badly seated head would be if the engine had been thrown together, if you have a quality recon unit it's unlikely to be the cause.
Lapping the barrels to the head is very strait forward and can correct a minor warp.
Put some valve grinding paste on the end of the barrel insert into the head and rotate back and forward. You only want to lap enough to get an even bright surface on the head.
Lapping will only correct minor problem with the head to barrel seat, any major problem and you will need to either replace the head or get the contact faces machined (fly cut). Any good engine machining company should be able to do this for you.
I hope its only a loose head, remember max torque on the head studs is 23 foot pound (assuming a type 1 not type 4 engine). You will need to check the torque sequence there should be some thing on line some where.
Just to double check you are sure it the head that leaking, it just that very worn valve guides also squeak and can sound like it the head leaking, if your valve guides are very worn you should get blue smoke on start up after the van has stood for overnight.
