another possibility, a bit of a long shot, and might lead to extra work for nothing ?
do you know in what way the engine seized ?
remove the head & see if the problem's in there. if so, the bottom of the engine will then turn. if not, remove the sump, you should get access to the bottom of the connecting rods, unbolt them & free the pistons. if one of them has welded itself to the bore, you may be able to turn the crankshaft away from the stuck piston.
i've used this method twice (not on alfas !) with a 50/50 success rate.
the first time worked, but on the second, the crankshaft itself had seized solid, meaning i'd totally wasted my time !
a slightly more drastic option, maybe a last resort !
if you do decide to drop the sump, and still can't get the crank moving, then the engine's already knackered, see if you can get in with an angle grinder, arm yourself with a couple of cutting discs & some goggles & see if you can cut through the crankshaft.
never tried this, and don't know of anyone who has, so i can't guarantee it will work ? there is a chance that the metal will be too tough.
might be worth a shot if nothing else will work ? i'd advise care in deciding which place to cut tho ! don't damage anything you might need later !
there may of course be a really simple way which i've overlooked, i'm willing to be corrected by anybody who knows better !