Going by the ford manual its 138.68mm

But plenty of people use them upto 138mm on modified engine with no issues

Findind a good cylinder head can be hard but there are out there, + loads of people think they are worth a fortune when in real terms they ain't,

I've seen some breakers commanding £600 upwards for a used head with usable deck height then add onto of that the cost of having it rebuilt, new guide, seals, ETC it becomes very costly

If you are really stuck i know of a 4x4 head with a deck height of 138.7mm that needs 1 valve and a set of guides and its in top condition no rot around the water ways