sorted
the pulley bolts were rounded, well all bar 2 of them, so hammered a 11mm hex socket on and they all came off which revealed the main nut. I was hanging off a large piece of scaffold about 5 ft ong on the end of a heavy duty socket and the block was on a engine stand with the flywheel end locked and had another full 3.5l engine on the legs of the stand as a counter weight and it did not move at all, propane heat, air gun nothing nada. ended up dremeling a groove in the nut as far across it as possible without reaching the threads of the crank and using a cold chisel and lots of force to provide a couple of thou stretch on the nut which them gave up its grip on the thread and came off as thought there was no problem at all. Now I can strip this beat of and engine and work out what to do with it
Next questions is can I go with a later larger valve head and skim the head to lower the compression and bring it back up to the M90 comp ratio, or do I need to get some gas flowing and bigger valves in the M90 head. What is the best approach for this?