There will be three threaded holes for "Jack Bolts" for jacking the clamping ring from the flywheel cover.
Blast the holes out, clean them and lube them.
Then install three 3/8" NC grade 8 bolts, and tighten them equally.
This should jack it away from the flywheel cover.
If the clamping ring does not want to move, then the next step is to drill a series of holes in it so that it can be split and sacrificed.
Once split, the corrosion will free up, and the ring can be removed.
They are not expensive in the used market......, and this is often the procedure used to remove a corroded one.
You can find them on e-Bay for around $30-40 or so.
Don't forget that the rubber cushions will have taken a "set", and that at least one (the inboard cushion) must be new in order for a good seal to occur.
(only the fwd cushion provides the seal between the F/C and transom shield)
When going back together, make sure that the exhaust manifold and the exhaust down tube align before the final tightening of the clamping ring.
(this is primarily for the cantilever hung engines)
Not normally an issue for engines using fwd engine mounts.
Be sure to replace the PDS bearing and seals, and pre-fill the grease cavity.
