Did you try breaking it loose with the starter? Use a socket on a breaker bar, with the end of the bar wedged against the floor or a solid metal part of the engine compartment, (remember which direction the crankshaft turns, and locate your bracing point accordingly) and hit the starter. This has always worked for me, on everything from a B18 to a B230. Shouldn't make any difference that the head is off, though you will need to be extra careful that no wires are touching anything, and shorting to ground. (less of a problem on an older car without FI) Hit the starter quickly and release---don't hold it for any length of time if it doesn't break the bolt loose. Try it several times---its the sudden impact that does the job, not the torque of the motor after it is fully engaged, and continuing to give it juice if it doesn't turn will oveheat the motor and can burn it out if continued for too long. Several quick jolts will not do it any harm.