[Mart]

It's hard to tell from the photo, but it does not look full of rust and crud in the bores, they look quite clean. So if stuck, it may not have been stuck for long. Again, hard to tell from the pic, but it looks like it may be relieved. My relieved blocks are sort of machined out with a parallel sided cut. That looks like it is shaped to the gasket, ie hand done.
I may be imagining the relief, hard to tell from the pics.

I'd be pulling the starter and pumps, manifold and distributor. Anything to take weight off it. You have already been told about the dangers of supporting on the early bellhousing.

If you pull the sump you can lever against the ring gear teeth to try and turn it.

Be very careful though, it's easy to overdo it, an engine on a stand is very top-heavy.

Mart.

Edit: Ignore my ramblings re the relief, the gasket is still on, isn't it?

Oh yeah, measured the bore yet? A rough measurement should be good enough to tell the original size.