Firstly, the belt and braces apprach. I'd check the block and head and bolts for straightness with one of those expensive 1 meter straight edges. Make certain the threads are tightened with a fully lubricated thread, but not dripping wet with oil. Use the right kind of ARP head studs with 3mm flat washers. Somewhere on FSPP, there is a reference.
Other Anaylisis and thoughts :idea: .
Like the Capri 3000 V6 Essex engine, the small Ford I6 has rear cylinders which are hurt by detonation more than any other cylinders.Due primarily to combustion heat, but also leaness and heat from the exhast. Front cylinder is way too cool, the back ones way too hot. Often, the back of the block is full of up to half a litre of old iron rust when flushed, even more when treated with custic or mutraic acid. Thats about 3 kilos of iron just floating around there! It stays at the back of the block, and things heat up there. In 1988, Ford OZ re-routed the water feeds and lines on the OHC to balance out the cylinder temperatures By adding an external line.
The rear water passages since the first 1959 144 ohv to the last 1992 XF 4.1 have two big holes. They are there to cool the back cylinder. In practice, there MAY not be enough dimensional stability under load.
Your only options are to check block trueness, and then look at the best composite gasket. O ringing the block is a good option, but it works best with a steel/composite gasket within the 0-ring biteing around the steel fire ring.
O rings aren't perfect, but the right kit can do the job and give you some peace. The x-flow blocks have a heack of a lot more meat in them, but the head gasket sealing on Ford sixes is the weakest point