seized slant six

I kind of agree with what the MoparMan laid out. I'd go straight to the Marvel Mystery Oil and put enough in each cylinder to make sure the entire top of the piston is covered. Let set overnight. Bump with breaker bar. Be prepared to deal with what ever comes out of the spark plug holes. Recommend a big sheet metal drain pan.

You'll probably wind up with scored cylinder walls, so a rebuild and honed bores will be the minimum necessary. You can run it for a while, but the engine may not get proper compression or have effective oil control. Hold off getting a rebuild kit until you find out if the block will need an overbore or not. If on the other hand, you get good compression and the engine doesn't smoke like an old outboard motor, pat yourself on the back and enjoy. Of course, once you know the outcome, change the engine fluids while the engine is fairly warm.