Welcome to the forum. There's lots of good advice to be had here.
Your cylinder to head Fasteners
screw into bronze thread bushings so are not likely to be seized, just good and tight. They are counter clockwise rotation to remove, looking from the underside. I would try a good quality box end wrench
(ring spanner) and an extender on the short bolts. The easy way is to link two wrenches
together, open end to box end. You will need to apply 30, maybe 40 foot pounds of force or even more. You will have to use an open end on the long bolts so it needs to be a good, stout one or maybe two weaker ones used together. (Same force as the short bolts.)
One small word of caution about removing the abutment sleeves that thread into the crankcases. They will usually be tight and often require heat to get them to turn. A further hazard is this. The threads are undercut under the ledge that seats on the surface of the crankcase mouth.
This undercut can act to break the edges of its hole while being unscrewed such that it can't be properly tightened in again without a level surface to seat against. That ledge is quite narrow.
Repair is a machining operation so it's not the end of the world, but you won't be happy about it.