Fill it with diesel fuel literally. Let it sit and once every few days bump the crank with a breaker bar. Saw a guy do that to a 289 in a couger. Didn't even rebuild it. Got it freed up drained it put oil and filter and drove it.
An old machinist told me to use diesel and acetone..nail polish remover... Buy it at dollar store...said he used this to unfreeze old Tractor motors that had been sitting out.. I tried it on 3 sets of gears I bought real cheap... IT WORKED !
Put the block in a heavy leaf bag or container mix 8 oz or so with 1/2 gal of diesel ..pour it down the cylanders...seal it up tight and let it sit for a week or 2
In the sun if possible..
There may be a modern a modern solution for 100x the price ?