Yes, I mean saturate it. If you have the back plate and face plate off you can access everything there is on a 66.
Lay it on it's front and saturate that rod connection and the fork next to it.
The same with the bobbin area. If it's that rusty it will take time.
Start wiggling everything from the needle bar back. If it wiggles it's most likely not stuck so move on till you find something that doesn't wiggle.
My 99K was rusted up tight, but 99% of the problem was in the bobbin area. You'll have to take everything that comes out, out so you can clean, so take it out now.
The hook has a big shaft that can rust tight. Attached to it is an arm that pivots on a BIG bolt. That can rust up tight. That arm was what hung my 99K up for so long.
Get the Tri-Flow, Liquid Wrench, kerosene, or what ever you are using in there and keep the area wet until you get it free.
Joe